Not sure I follow...
Do you mean that you have a user (ted) that belongs in a group (sales) and you want to remove that user from that group?
usermod -G sales ted
OR, do you mean that you have a group (superusers) that the user belongs to that you want to give the group permissions on a service, but not limit it to just a user:
chgrp superusers /sbin/shutdown
Could you clarify what you want so we/I can give you a more appropriate answer?
Cool