The eleventh (and final) wave in multiplayer involves getting to an extraction zone before the time limit expires. What happens if there are players outside of the zone when the time limit ends? Do all players need to be inside or just one?
Once you've reached Wave 11 (The "Extraction" wave), you've officially won the match, and are playing in a "bonus" round of sorts.
The only thing at stake is your extraction bonus, which tops out at "Full Extraction".
The only thing that matters is where players are when the timer reaches zero. If they're alive, but outside the extraction zone, they don't count. If they're downed an in the extraction zone, they don't count. Only living characters within the extraction zone count towards this bonus.
So you don't need anybody in the extraction zone, but the more you have, the merrier.