Conditions -> And -> Status of Player.
Also, if you don't want conditions but rather Player Groups, use Player Leaves event to add/remove players from a group, whether it be Left or Playing.
Also I think this belongs in the Trigger section. Galaxy Scripting is the JASS equivalent of Warcraft III's World Editor... or in other words, a non-GUI programming language.

LesTadO Regular ShmoeObviously that isn't what the Condition is called, but that's what I want it to check for. I'm trying to adjust difficulty in game by number of players. How would I do this? I am not familiar with Galaxy editor yet as I am still new to Starcraft 2. I used to use mapforge for BW and it was not that difficult to do in that.