The success of any MeetUp is really about the organizer ( Westerville 40's = the organizer is not kind, graceful, nor diplomatic) and of course the people who join the group. But again, if the organizer is accepting, treats everyone equally, and doesn't promote gossip then it's likely the group will be successful. If an organizer has pure intentions of getting people together we must assume that people are adults... don't micromanage!
In my somewhat limited experience the best groups are the ones which draw sports minded people together, such as kayaking or cycling. As a cautionary note it is probably best to never use your full name nor tell where you live or what your profession is. There will always be creepy people and if someone messages you via the MeetUp site-- simply ignore them!