I'm not the hostess, but I think I can help you out. You do not need to send a priority mail envelope. The hostess will provide that.
You should, however, put all your groups (all your quart sized baggies), into a gallon-sized bag. Your groups may fit into a smaller bag, but other people might have larger items (larger bags) and everything you are getting back may not fit into the bag you provided if it is smaller. It makes it easier for the hostess to be dealing with gallon bags for everyone.
HTH!