Sorry, I should have read the diff before replying to my emails. I think what Russell Is saying makes sense. If an identified group includes a members property in a statement, it doesn't imply that it's an exhaustive list or even that those agents belong to that group outside the context of the statement. 

There may be a use case for allowing multiple groups as the actor of the statement (or anywhere else) but I see this as a list of agents  and groups rather than a group of agents and groups. This use case is probably best dealt with using multiple statements. Marketing attended meeting 567,HR attended meeting 567 etc.

Let me know if I'm missing something.