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.