Married I agree with Elephants - if you don't know them from the group, don't add them. And even if they are in the group but you don't want to share things with them - don't add them.
What you put into FB is what you get out. If all you want it for is to get you through this meet up - you are in the group now, which is what you need. Make no friends, share no statuses/statii or pictures and you don't have to worry about anything. If you want to keep it going after the meetup, take it slowly, read your news feed and see what other people are sharing.
I'm adding everyone - as I have a FB account just for MN friends, so I don't know very many of them at all, but I only post things on there I'd post on MN anyway. But if people don't accept me as their FB is full of their real life, i'm not going to be insulted. :)