The gpasswd command (As defined in the man page) is used to administer /etc/group, and /etc/gshadow. Every group can have administrators, members and a password.
Command Options
-a, --add user Add the user to the named group.
-d, --delete user Remove the user from the named group.
-A, --administrators user,... Set the list of administrative users.
-M, --members user,... Set the list of group members.
gpasswd examples
Let us assume that we have a group named sales and three users user1,user2,user3.
Add User
To add user1 to sales group
gpasswd -a user1 sales
Add multiple user
To add multiple users to sales group
gpasswd -M user1,user2,user3 sales
Remove user
To remove user2 from sales group
gpasswd -d user2 sales
How to set a user as group administrator
gpasswd -A user group
or
gpasswd -A user1,user2 sales
Example :
gpasswd -A nixsavy linuxgroup
Set password for group
$ gpasswd linuxgroup
Changing the password for group linuxgroup
New Password:
Re-enter new password:
The example for -M above implies adding user1,user2,user3 in addition to users already there.
The man pages for gpasswd clearly state that option -M is to "Set the list of group members."
This will destroy any existing set of members, so therefore is not an implicit "add" unless the group was empty to begin with.