I don't understand the question, perhaps you're trying to make another group that has same user that are already on other group?
If that so, there isn't any group inheritance (that I'm aware of) so what I would do is to make a new group and add members to it, since a user can have many groups besides it's initial group
usermod -a -G newgroup someuser
usermod -a -G newgroup someotheruser
changing users an groups names as needed