Most progress is accomplished in small groups: There is strong consensus that group discussions rarely result in updating, even if they are fun. Conversations of 2 or 3 (maybe 4 at the most), seem to produce the most useful insights. This is why spending time together bilaterally is incredibly important to group development. When a handful of people are all interested in a particular topic and practice it together, they form a de facto working group which allows them to iterate rapidly and then teach it to the rest of the members.

This is important and oft-overlooked. Particularly for the more introverted sorts, smaller group = win. In my experience, large meetups (10, 20, or even more people) are a lot less satisfying than smaller groups. Perhaps four or five people is ideal.

Speaking broadly, there seems to be an inverse relationship between conversation quality and group size.