We are currently working on a 'Review Pools' feature for CodeCollaborator coming out in a future release that will be just what you are looking for.
In the meantime, you can approximate the behavior by creating a CodeCollaborator user whose email address is actually a mailing list of the people you would want to invite to the review. This "user" is the one you should assign to the review. When the review starts, everyone gets the email. Whoever wants to "take" the review (and it can be more than one) will go into the review, click "edit" on the participant list, and change out the pool user for their own username. Then they can do the review, and when they're finished, the review is done.
This is best because you have the log of who it was that did the review. More than one CAN do it if they wish, which is a nice option.
There are no licensing problems because no one will actually log in as the pool user. If you don't log in, you don't count against licensing (in either fixed or floating), so although there's this user in the user list it never counts towards licensing!
