If you need to add existing user to existing group use usermod command.
1. Check current user groups using id command:
# id serveradmin
uid=10001(serveradmin) gid=10001(serveradmin) groups=10001(serveradmin)
2. Add existing user to group using usermod command:
# usermod -a -G wheel serveradmin
3. Check if user has been added to group using id command:
# id serveradmin
uid=10001(serveradmin) gid=10001(serveradmin) groups=10001(serveradmin),11(wheel)
