The groupdel command deletes an existing group.

# groupdel friends

Before doing this, it’s a good idea to identify all files that have their group ID set to the given group, so you can deal with them later:

# find / -group friends -print

because groupdel does not change the group ownership of any files. It simply removes the group name from the system’s records. If you list such files, you’ll see a numeric group ID in place of a group name.