Possible different context: It has been made clear that the dungeons require a specific number of people in the party to enter. The R30 dungeon MUST have 4 party members. Not 3, not 5.

Perhaps this reservation system involves you being unable to book to a spot in the dungeon, then go off and do something else till it's ready to go. First 4 people to arrive? In they go, keeping it simple. Of course this has it's own set of implications for existing parties.

Just thought I'd throw a curve-ball at what might be plausible...