Consider renting out "Ambergris Dive Resort" (formerly Sundiver) - it has accommodations for 20-24 people, a beach bar, a pool and a restaurant.
Rates are very reasonable - probably the best deal going for such a group.
Lovely beach area, beach toys and volleyball. Easy access to the reef for snorkeling.
Perfect venue not only for all the celebrations, but for the wedding itself ......?
Location is not close to town, but Coastal Express calls at the pier every hour or so, and the road to town is fine from this location so golf carts work if you want to go to town that way. There is a small grocery behind the resort with a great little outdoor cafe, and you are walking distance to some fine dining - Mata Chica, Portofino and Rojo to the north - Rendezvous and "O" to the south.