Nothing says that you 'can't.

Is teamwork possible on summoning / binding.

If so for the 'summoning / binding test' or could dice be used for the drain test?

Even more then ritual sorcery, folklore is full of ritual summonings, so there should be a way in my opionon.

(though potentially the biggest exploit ever, since spirits seem to be one of the bigger explots as it is)

Sounds interesting. I would assume that only the leader of the teamwork would get the services of the spirit.

In past games using 3 ed, the group I played in did this alot, it works real well. Made enchanting under the old rules much more fun. We had a hermetic, shaman, enchanting adept, and elemental adept .