ALTER GROUP name ADD USER username [, ... ] ALTER GROUP name DROP USER username [, ... ]
This parameter must contain the name of an already-existing group to alter.
Use this parameter to specify the names of existing users you wish to be added or removed from the specified group.
Database superusers can use the ALTER GROUP command to add and remove specified users from a group. This is independent of the CREATE USER and DROP USER set of commands, and does not use them to remove the specified user account if dropped from a group; likewise, when you add a user to a group the account must already be in existence, as this command won't create a user account if the account is not found.
The following example adds the user accounts jessica and william to the sales group.
ALTER GROUP sales ADD USER jessica, william;
Then we remove the newly added users.
ALTER GROUP sales DROP USER jessica, william;