-- add users to a group or remove users from a group
ALTER GROUP name ADD USER username [, ... ]
ALTER GROUP name DROP USER username [, ... ]
The name of the group to modify.
Users which are to be added or removed from the group. The user
names must exist.
Message returned if the alteration was successful.
ALTER GROUP is used to add or remove users from a
group. Only database superusers can use this command.
Adding a user to a group does not create the user. Similarly, removing
a user from a group does not drop the user itself.