Just make it simple. If they're there on the day and assist, when the accommodation thread for the next DECA rocks up any volunteers from the last DECA are able to reserve a place in that same thread. They can't pay or fill out forms till the official thread and forms are available, but their name appears on the list with Ryan, Leon, Clive, etc.
If they don't reserve themselves between the accommodation thread and the official DECA thread they forfeit their priority entry and have to line up like everyone else.
If they don't pay within a couple of weeks, they get bumped off the list.