If the number of extensions in the group is less than 8, you could create a phantom user with the extension number of the group and then put cover buttons for that phantom user onto the extensions that need to cover the group. It's not the best, but it works. I have a car dealership set-up like this with "used sales" and "new sales" ringing the same group of phones, but on different coverage buttons so the user knows how to answer the call.