When looking for a group to join on MeetUp.com, there are two things you should consider. First, how many members does a group have? The more members there are, the better. You want to look for groups that have at least 100 members. This suggests that the group organizer stays active and engages with the members. Another thing to look at is past events. How often does this group meet? How many people actually attend? Are there comments left about how much fun the event was? These are important factors to consider. You don't want to join a group that meets infrequently and has poor attendance. Try to join a group that has an event at least twice a month. That way you will have multiple opportunities to meet other like-minded people.
Source: http://www.meetup.com/find/