Agents in anonymous group

Proposed fix for #353 . Let's leave the discussion of this PR in the issue thread to avoid discussing in two places.  
I think this advice is counter to how Groups are intended for use in the xAPI. If only 4 ppl did something, then some identified group they're in of 20 ppl where 16 of them didn't do anything related to the Statement is *not* the right actor for a Statement. When a Group is used as an actor, the meaning is the Group did something. If the meaning is not that the Group did something, don't use that Group as the actor. And I don't think we want to get into things like "well, what if some ppl were the main ppl and others were more peripheral" -- that sort of distinction should be in the meaning of verbs and activities and so forth in vocabularies.