gpasswd(1) on Linux
The text 3c989df2effc, as Debian 7 (Wheezy) shipped it.
administer /etc/group and /etc/gshadow
Flags it lists
-
--add -
--administrators -
--delete -
--help -
--members -
--remove-password -
--restrict -
--root -
-A -
-M -
-Q -
-R -
-a -
-d -
-h -
-r