Does "Group Message" mean that all recipients would "see" each other?
In my experience I always see all recipients in a group iMessage - so depending on if the author of the message chose email or phone numbers I see each recipient's information.

If I have that address/phone in my address book, then I get my address book's name for the contact, but even then, I can tap on the name and determine if an email or phone number was used to address the group.

On the iPhone, there is a group message toggle - but at iOS 5.1.1, it doesn't seem to matter whether this is on or off on the sender or the receiver - I still see everyone in the group iMessage. As far as I can tell, that switch doesn't do anything on an iPhone and isn't there to even help on an iPad / iPod touch.