AlterEgo -- that is actually incorrect on a few counts.
Once paid, group tickets are virtually always subject to the same "no refund, no exchange" policies that apply to single tickets. There is no "return basis". Therefore, groups that have purchased tickets for "Addams" or any other show are stuck with those seats once they are paid for in full. They are not able to return them to the box office post-reviews.
What I believe you are referring to is what is commonly known as "Unpaid Groups", a dollar value which is typically comprised of inquiries by group agents for potential bookings which are as yet unconfirmed by the box office, bookings confirmed by the box office to the group sales agents which require, but do not yet have, a deposit payment or payment in full by a certain deadline, and bookings that have been confirmed and partially paid but are not yet fully paid.
However, a truthful (or at least accurate) producer would not typically include Unpaid Groups in a representation of a show's advance since, as you've noted, that is not "real" money (until the tickets represented by those bookings are paid in full). Yes, producers do often lump in Unpaid Groups with their actual paid advance sales when they are specifically seeking to trumpet their advance (for whatever reason) and make it seem as high as possible. In this instance, however, the figure comes from Riedel so it's really impossible to know what he is or is not including in the figure (or whether he or his sources even know the difference), let alone if it is even accurate.
Either way, though, even if the advance is indeed $15M and even if it does include Unpaid Groups, I doubt that more than 10-15% of that figure is represented by Unpaid Groups, and likely less. The show has been on sale for many months now and typical industry practice would dictate that pending groups would have to cough up their dough or risk losing their bookings by or before first preview or Opening Night. So at this stage of the game, a show's Unpaid Groups would likely be quite low. Those that wanted the show so badly that they were willing to pay for it sight unseen have paid and those that haven't booked yet are either waiting to see it themselves or waiting for reviews. Plus, even if the Unpaid Groups are in fact 10-15%, that still leaves them with a single ticket advance of well over $12M -- nothing to sneeze at (though on some level also meaningless as described earlier in this thread).
"No matter how much you want the part, never let 'em see you sweat." -- Old Dry Idea commercial