Open Event to others after defined time

Looking for the concept of Subs in Events to fill last minute slots -- We use Events with a group of members for weekly scheduled play and are looking to be able to allow people outside the group to register for an  event at the last minute ( fill open slots)  -- thinking the Group can register X Days prior to the event -- then if not full Y days before the event its opened up to the public or another restricted set  of members ( another group , skill  level  , gender etc).