From The TimeSplitters Wiki

Trooper Grey Team
Requirements	- TimeSplitters 2 - Team Deathmatch - Works best with other player
Finder	Elite Yoshi

Trooper Grey team is a neutral team, the members of this team do not apper on radar, and the rank icon is grey during team deathmatch. This does not work in Deathmacth, Monkey assistant, flame games, bag tag, or Gladiator. To activate this, press the triangle button (or any exit button like the B button) right before the loading screen appears (This is after everyone selects their Character). If you do this right, your screen will have the character selection list in the background, but the loading will keep on going.

This is one of the hardest tricks to perform and works the best when playing with another human.