Simple.
Imagine it to be a ride where only 2 adults, 1 kid and 1 teen can go in. (2 dps, 1 healer, 1 tank)
People queuing will be placed into a queue on a first come first serve basis, divided up based on what you queued as. So 3 different queues.
When enough people meet the requirement, the ride takes off.
(The "ride" is the dungeon)
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