It should be the placement order from lead to 5th spot... this is why some geo teams are tough and some are easy to kill... same goes with clones... placement order is important but this wasn't ever thought out by the devs...
the game appears to go through team event motions like revives and assists based on ally spot. This is why if 5s is in the second slot he wont sacrifice himself if someone in the 4th slot dies (because when game checks for if ally died no allies died because the game hasnt processed the info from the dead ally in the 4th spot at the time its processing 5s ability in the 2nd slot.)
Animations happen fast or slow but always begin at same time so fast animations will appear to land first but that doesn't mean that the working ability actually is applied in the same order as the animation occurs.
It would be nice for devs to confirm mass assists but I doubt they even know how it should work or how it works... they just happy it hasnt broke yet.