More than likely you'll want to do a loop to do this for each player. If I were doing this I'd create the units in the trigger at certain points, to me that would be easiest.
Note, If you do this "follow a unit group" action, the minimap will not be usable and I'd also recommend using the "lock camera input" under the camera section too, so the player cannot change the viewing angle of your perspective.
One thing. What would the event be for this trigger? Just do it at map start? I did a lot of sc1 modding and there was an always event which I haven't found in sc2. SO the event so something I want to always happen confuses me a bit.
sc1 editor had an always event I don't know what so hard to understand about that.
"Always select units in region one, issue order attack to point 1"
now in sc2 two I have to put
"every 0.00001 seconds select units in region one, issue order attack to point 1"
It sounds like from the above poster you have to program a loop, which is fine.