First think about what you can offer:
- Why should experts of any topic come to you?
- Why should they answer questions or discuss on your site?
- What is the advantage of being active on your site?
The first two questions can be easily answered: They should come to you because you can offer extraordinary content concerning the topic. To build this extraordinary content, you have to create content, and spread this content.
Content is created by users who aren't present right now. So you have to create some content with the users who are already there. Don't hesitate asking question you've seen before, do it. Content is crawled by search bots, they will place your content in their rankings. And even the stupidest question gets googled again and again. This is your opportunity to attract some users, but not experts. Most likely you gain users because they look for an answer to a question they can't answer themselves, or because they simply want to discuss the result. Since somebody in your community has experience concerning to this topic, you can answer "easy" questions resulting in even more content. Keep it on. At least, your site should have a few active users to create a healthy base. Theses users aren't many but they will generate what you're missing right now.
Post pictures concerning your topic. Don't underestimate it, it will give you a rapid boost. Whenever someone googles about your topic, he will be shown some links and pictures about it. Your pictures can rise way faster since the most pictures are crap anyway. Quality makes an important point here. Everybody can post crap pictures, but not everybody can post pictures of a good quality showing the topic as it is. The user will click on it, will be redirected to your site to see the picture in full resolution but he has to do a short stop on the page which shows the picture. That's the chance to keep him. If he's interested in the picture, why shouldn't he be interested in some written statements? Often, I see myself staying on a site way longer than I wanted to because the picture led me to a great site.
After you generated content, begin spreading it. Look for another site having the same troubles as you have. Do a partnership. Ask the same question but answer it differently, and link to the other answer. There's basically no reason to answer questions equally if you don't have any reputation. To make yourself attractive, you have to be different. Don't answer questions like everybody would do, do it like somebody awesome. Make it shiny! Try to make it extraordinary since this is what you need to attract and get some experts.
Another way to spread a site is to talk about it. Talk, talk, talk. Talk to your real friends, talk to your SE friends, talk to your Steam friends, and so on. Show them that they can critically influence the community because it hasn't really grown yet. Encourage them to ask on your site instead of googling. It may be easier and faster to google but they won't get the same quality answers. Post a status on Facebook saying you're on this site, that's enough. A lot of people will click it, share it, and maybe stay.
Now you've generated a lot of content but where are the experts? They will come. Users come, as I mentioned earlier on, because they want a question to be answered, experts come to answer and, furthermore, to discuss these. Users don't look for a community answering their question, they look for an answer. Experts look for communities to share their knowledge and to answer unanswered questions. Users don't browse to find you but to find your content, experts browse to find you. A lot of content means a lot of opportunities to participate as an expert, to get known as an expert. Experts don't just want to answer your question but they will also try to get a reputation so they are considered if it comes to harder questions.
So, why should the experts stay? You built up a strong content, and strong content requires a strong community. A strong community is your meat and potatoes. The advantage of being part of your community is simply to be part of it. You generate content that can be answered, you have a community, and you have other experts. They will most likely stay since they can
- discuss their view of point with people who are on the same level.
- find friends through your big community.
- earn a reputation which is useful in a big community.
Don't worry to much about the experts, they will come. Worry about the users and your content. In this case, the content definitely win. Content is traffic, traffic is the life of a site. Traffic lowers, site's dieing, traffic rises, site's growing.
Hope it was a little bit helpful.
Edit: I just saw that you've provided additional information about your site.
StackExchange sites have some things other sites don't have. Usually, users should be straightforward and answer questions as they are, they shall not discuss about it but show some worthy answers. This will deter users who want to discuss but in exchange you create a community where the answer is the main idea. You ask a question, and you get an answer as fast as possible. So, how do you attract users to a site where not the way to the answer but the answer itself is the center?
Basically, the idea behind it is great because you cut all the crap and get answers. Therefore, this idea, including all SE sites, should flourish since it's awesome to receive information and solutions to a question. But as you can see, they don't. Users are used to the usual forum about specific topics, e.g. gaming forums, drawing forums. The forum is built to primarily discuss the answer to end in an appropriate answer. Nevertheless, you always have an area for off-topic threads. If the users are bored of the main topic, they can just stay on the site and post their wish thread into the off-topic area. So they are still on the same site, with the same users, but the topic changed. The off-topic area is a great opportunity to get involved. Usually, you can
- Introduce yourself to the community.
- Find friends through threads like "Looking for gaming friends".
- Discuss any topic since it's off-topic.
- Play forum games.
StackExchange lacks these things, it's not built to do these things. You're here to get a straight answer to your question. So, to extend the base question, how do you attract users to your site that doesn't offer many opportunities to socialize?
If you don't have the opportunity to socialize, you have to fake one. If someone asks a question, you and multiple other users should answer. Every answer states another view of point although they may say the same. There's the point: The answer may be the same but every view is different, it reflects the uniqueness of every single user. So, if you have many answers, what does it state? It states that you have a lot of users besides the fact you're not a real forum. The users will most likely stay since they see a lot of participation and helpfulness. It can even result that they will answer other people's questions. A side effect is that you generate content that will attract even more users.
Make reference links. Everybody links to Wikipedia. If you are on any other board, what you most likely are, and you can grab the chance to talk about your topic, take it. Make a quick reference link to your SE site showing the power of knowledge. The advantage is that Wikipedia isn't a rated system as SE. You can upvote answer, so users see that you have a clue about it, it simply is impressive to see an upvoted answer. Experts will click this link and see a community about their topic with a reputation system. They are going to join.
Don't worry too much about your experts. At the beginning, you won't get "cool" questions, at least it's really unlikely. So, if you don't have "cool" questions, why do you need experts? Do you think that they want to answer stuff that can be basically answered by Google? No, they don't. But if you get a "cool" question, promote it like crazy. Tweet something like "We're having a hard time concerning this. Any suggestions?". Make the cool things even cooler, let everything else as it is. Promote your cool questions, and simply answer the uncool ones, but still answer those. Make a big deal about these questions since it will show that experts can answer hard questions on your site.
Getting users in general, including experts, is all about having good rankings on search engines lists, and being different than other. Keep it on, try to get a few questions every day. More content are more links linking to your site. So there are way more opportunities to get spotted by a random Google-er.
Never refer to Google. At the beginning, you are small. You can't afford loosing any user. Users come to you because they didn't find anything on Google, it doesn't mean there's nothing at Google, they simply didn't find it. Don't refer to Google then, even if it's the dumbest question you've ever seen. Answer it nicely and make him happy. He will recommend you then. And recommendations will keep you on track, and let you gain additional users.
Generating content on a beta SE site isn't easy, I have to admit that. But you have the users from the proposal, you have your site in the list of SE sites. If you talk about it whenever you have the opportunity to, you can't get on a road to hell. Actually, the SE community is the perfect place to get a site started since you can generate a lot of traffic just by being a subdomain. If you ask about every "shit" at the beginning, you can generate a lot of content just by doing so. Not everything can be on a professional level or an expert level, accept it. You can't expect your community to have "expert content" on the start. Your community is evolving, they will generate expert content if they have enough knowledge about the basic. But before, they have to get these.