I've written a ton of code for the game, and didn't discover the "Advanced" tab in the mission editor today.
;D You poor bastard, that's not funny at all .....
Depending on exactly what you want, there are two possible solutions. The first is Rocko's - group the trigger to the group and look at the new options in the Activation box.
If that doesn't work, you can use thislist. thislist is an array returned by a trigger and consisting of all the units in the trigger area that are on the "side" first mentioned in the Activaiton box. For example, if you set the Activation box to East not present, thislist would return an array of all the East units in the trigger area.
You can use thislist in the Condition line, for example:-
Condition: this and (loon1 in thislist)
By using the command list you can also use the array elsewhere, for example in a script. Note that the array is dynamic: if you want to "capture" it at a moment in time you need a + command. For example, I have a script with this code:
_trigList = list mapTrig
_upList = +_trigList
where mapTrig is the name of a trigger.