You'd have to trigger it.
Make a dummy ability "Add to unitgroup" or whatever.
When it's cast, add every unit selected who's not in the unitgroup to the unitgroup.
Then when you issue an order, have a trigger issue orders to all units of that group.
You could indicate which unitgroup a unit was in using different color selection circle model using buffs. Then just make it so that when you issue an order to a unit, it takes the unit's unitgroup and orders the entire group to do what you just ordered. But only for basic orders, or abilities they all have.
It can get a bit complicated, but I think it could be done fairly easily.