At what point is a group of people considered a community? Can a community consist of only two people or is there more required?
Some examples I'm curious about:
To me the point where a "group" becomes a "community" is when its able to create sustainable growth of membership simply through its activities. Actual membership of the community may have high turnover: new parents, babysitting co-ops, and sports leagues come to mind. However, the nature of the community is such that it continually renews membership, leaders, and elites.
Taking your examples above:
Blogs are most often themselves not communities. If the primary author were to stop creating content, then the group of people investing their time will likely dissipate as well. Related to this, The Apache Software Foundation considers the importance of one or a few committers to an open source software project to determine the health of a community. They call this the "bus" or "pony rule" — what if a bus were to take the few important members away? (See my tweet.)
An example of a sustainable community around written content would be fan fiction for Star Trek.
Github contributors become a community if a large enough mass of committers can sustain, or even direct a project even if the governing authority were to disappear. A great example of this is the Node.js community forking away from the community sponsors because they were unhappy with direction. See this article for latest status on this community drama.
Power users — if a group of users can, in and of themselves, create a sustainable community or organization without help or organization from the parent company of the product, they could be considered a community. An example of a very powerful one is ASUG (Americas SAP User Group). However, I believe that independent power user organizations are a construct that mostly formed pre-social media era.
Since social media era, most companies are creating their own power user "communities". These communities are not independent, but can be extremely large. Consider, for example, the SAP Community Network which is controlled by the company, but consists of well over 1 million users and implementors.
If the sponsoring company were to choose to pull the plug on this community, its possible it would have enough critical mass to reform under its own power, but in a severely limited way.
Everything described above is about critical mass of time, energy, community-owned content, and people. Very helpful is having renewable powerful forces of identity that would help potential community members identify that they should be part of this community — like people having children, or the involvement of institutions such as schools or churches.
A necessary precondition for a community as I understand it (conversely to a random group of people) to develop, are a certain amount of stability and continuity. The leadership and rules/policies should remain stable enough for the time necessary to build up a core of engaged long-term members, who help defending the goals and needs of the community.
Conversely to a random group of people, a community is characterized by some kind community spirit or feeling, and its members have a more or less pronounced tendency to care for each other.
In addition to the official rules and policies, the community may also develop informal norms that are not written down, but nevertheless have to be learned and understood by new members. These informal norms are not enforced by the leaders, but hold up by the community members for example by rating posts via up- and downvotes.
In a (rather democratic) community, the relationship between the leaders and members is binary, such that the leaders or founders determine the purpose of the community, but at the same time the needs of and feedback from the members are taken serious, or community members are even invited to help determine the rules and policies. Members are taken serious and respected, instead of getting considered to be random easily replaceable accounts that post stuff.
Looking at the examples mentioned in the question, around my favorite blog there exists a nice community of (concerning certain issue) like-minded people. There is a good core of long-term members, but of course there are also random commenters who pop up out of curiosity and disappear rather quickly, if their mindset or views do not resonate with the community. Unfortunately, the commenting system Disqus removed the possibility to downvote for unknown reasons some time ago. This significantly diminished the value of that community because since then, the only possibility to discourage comment(ers) that are not welcome (and their by the community not appreciated behavior), is the ban-hammer of the blog owner.
Users of a service, who for example face a technical problem and want to get it resolved, rather don't form a community as defined in this answer. Each user rather minds his own business and is primarily concerned with what he can gain from the service offered, he rather does not care for fellow users and their needs. The relationship between a service and its users seems to be rather one-way: the service (under certain terms of service) offers something the users need, but the users have rather no (direct) influence on the rules or terms of service.
Concerning co-workers in training for a product launch, already the definition says that they (need to) work together. To be successful, they better maintain a good work (or community) atmosphere, figure out in common efforts how to best achieve the goal of successfully launching the product, constructively resolve conflicts and disagreements, deal with unpredicted eventualities, etc. So I would say a group with a common goal to a achieve, such as successfully launching a product for example, rather has to form a (maybe small finite life-time) community as defined in this answer. If these co-workers work for the same company, the stability and continuity of general rules and leadership might be provided by the employer.
Taking a terminology angle. Some picks from the dictionary definitions:
a social unit of any size that shares common values
a unified body of individuals
— Community in the Merriam-Webster Dictionary
A group of people living in the same place or having a particular characteristic in common
— Community in the Oxford English Dictionary
In the plain language sense of the word, clearly a group of people becomes a community when they share something. Anything will do.
Hence I think all of your examples can be seen as communities. (For example, by the dictionary definition, it is correct to call the group of people who share an interest in your blog your reader community.) They might not be valuable or interesting communities unless their participants communicate somehow, but they're communities nonetheless.
Borrowing a concept from marketing, maybe a community is a group of people who reference (communicate with) each other.
So, on-going in-person groups are usually communities, like a classroom or workplace. You can have a community within a community, like a class within a grade within a school. They don't have to be in agreement to be a community, but do need some underlying agreement, like a political party within a parliament.
All the people who watch The Simpsons aren't a community, but a group who discuss it are.
And here it gets a bit tricky: a website like reddit is more like a TV show in that most people just consume content. OTOH, if they write a comment that is popular, they are more like a writer of a hit TV show. But, in the subreddits, if it gets small enough, then there might be a community amongst the people active in discussion.
I think there's a penumbra of community around the core back-and-forth discussion, including people who are often participants, but not at this moment; to people who potentially particiate.
Perhaps it's enough to be a member to listen and take seriously what others say (listenting is "communicating"), which is more than being a mere watcher of a TV show. But I think there must be a real potential of participating in the discussion.
BTW: this concept in marketing is in contrast to the idea of a market as a demographic of people with some common attributes, needs, interests. Instead, for marketing, the point of referencing each other is that information is passed around the group (aka word of mouth). But to me, this also seems to be the essence of a community - wherein people "commune".
it depends on who's asking. There R many definitions (mrkting pros, community organizers, politicians, etc).
For me it is when there is wide agreement (w/in at 25% - 75%)that there isa shared identity, one is able to make meta comments abt the social grouping ("they xxx"; "we xxxx."), an almost semipermeable membrane bounding the enclave (community can not B applied to mechanical systems). A network is a looser affiliation and does not have this shared identity. Community has been thought of land based but now can B seen as "community of the mind" (I would like the older definition as things begin to stretch a lill too much).
It takes only agreement between 2 or 3 externals to ID a community as existing. In someways this can help a community beyond the tipping point (self identify). "Others say it, we accept it." in THIS sorta 2 step process (not the only method)...
