The few group games that I've run in the past I've found it good to have a Main Game Thread and then a few key place threads. Like have a thread for each private Common Room, a Hogsmeade thread (since they are seventh years they would be allowed there on the trips down), a Potions Room thread (cause that's always a nice, dark place for some fun >.>), maybe even a Shrieking Shack for the Marauders to sneak off to...just some ideas.