Semporna is not an island. It's a small town about 80km away from Tawau. You probably mean those island surrounding the town i.e the North western side - Tun Sangkaran Dandai Marine park and also the South Eastern islands of Mabul-Kapalai & Sipadan ?

Anyway, good idea of going in a group of 5.. Infact if can get 8 much better bcos going to those islands you need to hire a boat (either illegally or via proper licensed agents) Each boat can accomiodate max of ~ 8 so just nice to share the cost.

If illegal boat operators, you can get it for ~ RM350 to RM500/boat per day (RM350 to Sangkaran Dandai side & RM500 to Mabul/Sipadan side. If oing via licensed agents, you need to X 1.5 to X2. On budget hotels, there are few in Semporna ~ RM40/pax by sharing in the hostel room