This post isn't intended to be anything stunningly original, but simply to collect a few basic, but under-utilized observations together in one place.
The question of how we can progress human knowledge and understanding is one that is of particular interest to people on this website. The most obvious reason why groups are important for these purposes is that often a lot of work is required to move a field forward and when more people are working on a problem, you might expect that more work would get done. There are other reasons too, but this the main one.
This immediately leads us to the question of what factors are important for allowing a group to produce knowledge:
Anyway, I hope that this post helps you think about what kinds of groups ought to exist.
However, there can be diseconomies of scale where it takes more time to follow all of the work that is being done and more time to filter out the good work from the bad.
As an example, Effective Altruism tries harder to be friendlier as recruiting more members arguably allows it to increase its impact. On the other hand, the Less Wrong community is less desirous of growth, lest it lower intellectual standards.