I'm the lone moderator of a relatively low-activity, but large-membership, Facebook group.
What strategies work well for recruiting other moderators?
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I have had quite some experience with moderation and administration. Here is how I would approach it:
1. First Option: You have spotted members with a potential fit for the job:
Contact a couple of these, ask them to join you on this moderation job and give them a month or so of trial with limited powers... keep the best ones, and kindly thank the others for their effort (if possible in any way, give them some reward or recognition if they are active members of the community as to avoid diplomatic incidents).
Pros: You directly reach out to members you know are capable of being serious and efficient from personal experience. They will also feel privileged to have received an invitation for the job and may be more eager to participate because of it.
Cons: You will need to say 'no' to some of them even after you had invited them to participate which could be tricky. Also, by manually selecting members to invite for this role you may be missing out on very capable members you haven't yet spotted.
2. Second Option: Reach out to the community - (my personal suggestion)
Make an announcement to the community that you are seeking one or two new moderators, include a brief job description and some sort of link to a form/application process. Make them write some paragraphs about their past experience, connection with the community, and why they would make good moderators.
Now, this writing stuff is mostly just to get rid of those who would just like to join for kicks... as soon as people actually have to commit and fill-out some sort of an application you know that those who do answer it seriously are those you should consider seriously.
Then again, give those you see fit a month or so of trial with limited powers, and keep only the best.
Pros: By appealing to the community, it is probable that only those interested will respond. Furthermore, you are potentially engaging members very capable of the job which you may not have noticed in the past. This will also make the community feel involved, even if they don't actually respond to the announcement: they've seen that you're appealing to them.
Cons: Not a fool-proof process as many will still fill out the application with limited seriousness. Costly in terms of time as you may potentially need to review lots of applications.
Now, before you do any of these you need to ask yourself:
Do I really need more moderators? You say this is a "low-activity, but large-membership facebook group": If there is low activity, there should be a low supply of work for moderators... although, as underlined by Cyn, it can be good to think ahead.
Do I have the time to supervise and judge these candidates? It is important to be on top of things for the trial period... not in that you need to constantly be behind them - on the contrary, see how they behave on their own - but you need to be there in case they go off-route and also to evaluate their competence.
At the end of the day, there really isn't one way to do it, it's just about finding the right one for the context of your community.
You are wise to bring more moderators on board before you need them. The list is low-activity for now but that could change in a heartbeat, and often does when there are a lot of members. Or you could be out of town, have computer problems, or be sick, just when something blows up.
The user Nobody has given some excellent advice but I prefer reaching out to selected people vs putting out a call for volunteers. The latter is great if you get lots of responses and then have to narrow them down. If people are eager to do the job, let them show you what they can do. In my experience though, asking for volunteers brings few or even no responses and some of the responses will be people you don't know well enough to trust with access or people you already know aren't suitable.
Facebook does have admins vs moderators, where moderators can't change settings or kick off other moderators (or add new ones). That helps. But you still want someone who you know is going to act reasonably.
You might do both methods, then make sure the number of new moderators is at least one fewer than the number of applicants, if you have to cull anyone but don't want to say why.