ijust made something called a massing wisp for my map
Its invulerable and invisible, with unlimited blink.
move it to the loacation you want then use a text cammand to trigger a picke every unit action, followed by a unit order issuing a point (the unit being picked unit, and the point being the location of the massing wisp)
this will move all units to the loaction of the massing wisp. But with a lot of units you may experience command lag.