Summon has the ability to turn a good 3 v 3 or 4 v 4 group fight into a succession of 4 v 1 ganks.

However, I do think it is a useful spell/supp.

So what I'd propose is that the more people from your cabal or group in the area with you, the greater the chance that your summon spell would backfire and send you to the person you are summoning instead. e.g. something like 20% chance of backfire for each ally in the area with you.

This would prevent it being used to gang as much, but keep it useful for simply getting to an opponent for a somewhat fair fight.