You need their ingame handle. The developer of Starcrafts mod made a map for people to get their SC2 handle
Here is a link to that video where he explains it.
Edit: Oh and once you have their handle you can loop through every player in the game and compare their handle and see if matches to the ones you have stored in your map
How to make a trigger run if a specific player is in the game.
@WesMan00: Go
You need their ingame handle. The developer of Starcrafts mod made a map for people to get their SC2 handle
Here is a link to that video where he explains it.
Edit: Oh and once you have their handle you can loop through every player in the game and compare their handle and see if matches to the ones you have stored in your map