I was looking for an answer on this and found that back in '06 someone asked the same question and was told this is not possible.
What I was looking to do was trigger damage to an entire team in CS. Although complicated to achieve, I wanted to share the info with those who may be looking to do this on a map.
First create a game_player_hurt and name it. Lets call this: damageall50
Set the damage to whatever you want. For the example I will say 50.
Now create a game_team_master and name it. Lets call it: damageall
Set its target to the player hurt: damageall50
Leave the team as -1 for all for purposes of this example.
Create your trigger_once or trigger_multiple brush entity and point it's target to the team master: damageall
Make your map and watch the magic when you walk over your trigger brush.
Despite the comment about it not being possible on the 1.6 engine. It can be done.