just to throw out an idea: 

while we wait for other people because they lost interest, why dont we start with something like, character development and training/sparring. which would accomplish two goals, IC would give the chance for characters who have not worked together time to develop as a team and learn what each other can do. with the added side effect to keep interest with those who have joined, and only start the plot proper once we actually have enough. 

just a thought. i dont know if that would fit into your plot at all, or if it could be made to fit.