Have all the group members to sign in into the jam? Or can be only one of them and then credit the rest?

Writing RPGs in groups is fairly common (Dungeons & Dragons and Pathfinder are 2 popular RPGs that are maintained by large groups of people).

For clarification: You can't make video games (since this jam is about pen and paper RPGs). You can still make a game, it just has to be a tabletop RPG.