ok i would make 2 arrays
arrayA is an array of all units, eg all six of them
arrayB, which needs to be updated every time a player respawns, is an array of players
Subtract all alive elements in array B from array A and voila you should have an array of ai units
Then join these units to a different group
There's probably a damn sight easier way to do it, but at the moment that is all i have to offer
arrayA - arrayB = arrayC