doesnt just have to be summons that can be done with way, certain spells could be activated this way too. lets face it, the conjurer flare ect is kinda lame, if it was activated as a BR spell and each person taking part increased a base damage my 10% 15% it could be quite dramatic with the right animation to back it up.

your idea has ALOT of potential. even melee attacks from BR.

imagine you have a party made up of 1 glad, 2 mrd, 3 pug, 2 mages, all the melee enter into a BR and a certain combination activates something like omnislash where each character taking part rushes to the target unleashes a hugely devestating blow and runs out past the other side.

so much potential with your idea i love it