From an e-mail that a friend sent:
When you change groups, all of your current credits are transfered to that group. If you remain a member of the old group as well, you also have the option of having only future credits go to the new group, or splitting all credits evenly between the groups.
He just switched groups, so I imagine that he's correct, but I have no idea of the accuracy of this information.
Also, I've had enough feedback that I tried to create the group and got a "Page cannot be displayed" error. In trying again, it said the group was already created. However, I believe you need a group ID to access it directly and I don't have that ID yet. Until the group lists are refreshed, I won't be able to post the URL, but I will do so as soon as I have the info.