There’s always the chance that everyone could be wearing costume pieces underneath their street clothes, although that would make Oliver’s statement about changing for 20 minutes all the more baffling.
At first, it looks like each person has their own changing room, but then Sara Lance opens her door and it’s just a closet with shelves.
Maybe he’s accounting for the time each hero had to spend closing their eyes and promising not to look while others changed?
Speaking of the randomness: why does this facility not have separate locker rooms?