You can get a list of all the units in a trigger area...
If you use the OnAct line:
thislist exec "trigger1.sqs"
To execute your script, you'll send the list off all units in the trigger radius to the script as _this.
Just something to note, the trigger must be set to a present condition to do this, and if you have it set to west present it will only 'see' west units, and east it will only 'see' east.
Anybody present means it will see all four sides (East, west, civvies and Resistance).