We can break into four categories:
1) We join groups where we know existing members. We get an invite from our friends and we join.
2) We join groups that are likely to increase our self-esteem. The group is doing important work, is exclusive, or has an impressive membership roster, so we join to feel better about ourselves.
3) We join groups with members most similar to us. People have a similar demographics, habits, or psychographics to us (usually a new passion) so we join because we're like them.
4) We join groups by pure chance. We stumble into a group by chance, so we join.
The first two are much stronger motivators than the latter two. We're far more likely to join a group as more of our friends join the group. We're far more likely to join groups if it makes us look and feel better about ourselves.
The problem is most community builders rely on the latter two. They stress and promote how similar the group is to prospective members or hope members stumble across it by search/chance.
if you want more members, either use your existing members or ensure the group increases the self-esteem of members.
If you want to learn how to attract sustainable growth in communities, come to SPRINT Europe next week. Tickets are still available.