The problem with using Google+ as a closed collaboration group, at least for the moment, is that circles are not persistent across users. In other words, I can create a private circle with +Deborah Mourey
and +Eric Miltsch
in it, along with others, but neither of you will know the name of my circle, nor will you be able to send a new message to it, without recreating the users in the circle on your end, by hand.
That is inconvenient, of course, and would make keeping circle members up to date across all users a real PITA.
No doubt Google will address this issue, because plenty of people are talking about it.
One potential solution I can see is the ability for users to "share" circles (meaning the list of people in the circle) with others, which means that the circle could automatically update as the original creator/sharer adds/deletes users.