Try doing the following things:
1. Name the group that contains the players. To name a group, put the following code in the initialization field of the group's leader:
squad1 = group this
2. In your trigger's Condition field, put the following code:
west countSide (units squad1) < 1
Change west to whatever side the units are on if they aren't on west.
This code will count how many units in the group "squad1" are alive on the west side. If there is less than 1 (i.e., 0) alive, then the trigger will activate.
If you want specifically to check if a unit is human controlled vs. AI controlled, you'll have to use a script for that. Let me know if that's what you want instead.