Assign an Activity to multiple Groups?

Just by the look of the data model, i guess the answer to my question is NO,
My question is if any one had this issue before and how was the work arround.
My best thougth is to create a AuxTable where I can say that the mega group that i create is composed by goup A and B. And group the mega group must have the users and permitions of both Groups.

Have any thougth of another posibility
No One? I really nead some ideias Here :(