In Guess Who!, players are split into two opposing teams.
One team hides and one team seeks.
Those hiding have to disguise themselves between NPC's. They have to act and look as natural as possible because the other team will seek them out.
The hiding team can change their disguise and follow the other team around, or they can take the easy way out and hide in the far corner of the map. The choice is theirs.
The hunting team has to find out which one is an NPC and which one is a player. It's a more active version of Prop Hunt. Instead of just standing still, you're expecting to act and look like an NPC.