A trigger returns an array called thislist which is an array of all the units in the trigger area that satisfy the first part of the Activation Box. For example, if you have West (doesn't matter whether it is West present, or not present, or whatever) then thislist would be an array of all West units in the trigger area.
You can then use this array in the Condition line. For example (syntax not guaranteed)
Activation Box: West present
Condition: loon1 in thislist
On Activation: as you like
If you group a trigger to a group leader (as if the trigger was a loon) then you get new options in the Activation Box, which you might find useful.
There are other types though I can't remember exactly what they are. The are in one of the comrefs somewhere.