There is lots of advice around on this & other forums (try searching here, PFA, Flyer etc.) about specifics to look for with regard to agreements & arrangements etc.. so I won't regurgitate it yet again.
My advice is to look at the people in the group first - they're the key and it has nothing to do with aircraft. If you feel comfortable with them then go into the detail - if you don't, stop right there & look for another.