A trigger is just a funning looking script that is always running. You can put anything in the Condition field that you would put in a script...the nice benefit is that triggers can give you an array of units in an area.
So...yes to your question.
If you want to fire a script when two items get close, use something like the following in a trigger:
Axis a/b/angle all 0
Activation: none
Type: none
Condition:
obj1 distance obj2<100
On Activation:
<put your script call or statements here>
Then, whenever the two objects are within 100 meters, in this example, whatever is in the "On Activation" field will happen.
If you something to happen when a specific unit or group enters a an area, set the trigger axis and angle to whatever you need, set the Activation to the unit/group's side, Present. In the Condition, put
<unit name> in thislist
for a unit check, or for a group check put
"group _x==<group name>" count thislist>0
Does that help?