Google Groups' different types of groups ( Web Forum, Email List and Collaborative Inbox and the other one ) do more to muddy the waters of understanding Google Groups than actually describe what it is they actually do. Anyway...
I previously have blogged here that Collaborative Inboxes didn't work because in order to receive a reply to an enquiry - it meant that you had to be a member of a group - and therefore able to see everyone else's enquiries. This is the equivalent of group therapy for a clap clinic, in that as a question asker, you get to see everyone else's questions.
I was only now, just showing someone interested in using Collaborative Inboxes, that particular quirk when I discovered a "cc The original sender" checkbox which is by default checked. Yay!
This means you could now use Collaborative Inboxes for private issues, that get responses from the experts who are members in that group. AND fellow experts can see your response. AND the person who replies doesn't have to remember to make sure the person who asked the question in the first place gets a reply.
This is great and means that Collaborative Inboxes can be used for generic enquiry email addresses and responded to by a team of people - which is what they were supposed to be in the place.