http://www.debianadmin.com/users-and-groups-administration-in-linux.html

Add New User in Linux

-d home directory
-s starting program (shell)
-p password
-g (primary group assigned to the users)
-G (Other groups the user belongs to)
-m (Create the user's home directory )

useradd -g test1 -G test2 -s /bin/bash -p xxxx -d/home/admin -m admin
This will create a new user admin.

One additional switch worth mentioning is "-D", which controls the defaults for useradd.
Specifying the "-D" switch on its own will simply display the default settings, while specifying -D in conjunction with other switches will change the defaults to those values.

adduser -- User Friendly Frontend for useradd command
adduser [options] user group

#adduser admin

Adding user `admin' ...
Adding new group `admin' (1001) ...
Adding new user `admin' (1001) with group `admin' ...
Creating home directory `/home/admin' ...
Copying files from `/etc/skel' ...
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
Changing the user information for admin
Enter the new value, or press ENTER for the default
Full Name []: Admin
Room Number []:
Work Phone []:
Home Phone []:
Other []:
Is the information correct? [y/N] y

You'll notice that, by default, the adduser command creates a group with the same name as the username, and makes this group the primary group for that user. This is called a user private group (UPG)

Modify User in Linux
usermod -- Modify a user account

Syntax
usermod [-c comment] [-d home_dir [-m]] [-e expire_date] [-f inactive_days]
 [-g initial_group] [-G group [,...]] [-l login_name] [-p passwd] [-s shell] [-u uid [-o]] [-L|-U] login

Options:
-d home directory
-s starting program (shell)
-p password
-g (primary group assigned to the users)
-G (Other groups the user belongs to)

To add the group ‘others' to the user admin
#usermod -G others admin

Remove user from group and add another group instead
groups pvcs
pvcs : pvcs wheel

remove group wheel and add oinstall
usermod -g pvcs -G oinstall pvcs
groups pvcs
pvcs : pvcs oinstall

Delete User in Linux

userdel -- Delete a user account and related files
Syntax

userdel [-r] login
If you want to know available options check userdel man page

Options
-r (remove home directory)

To remove the user ‘admin' and his home directory
#userdel -r admin

deluser -- remove a user from the system
deluser [options] user group

If you want more options check deluser man page

Example
By default, deluser will remove the user without removing the home directory, the mail spool or any other files on the system owned by the user. Removing the home directory and mail spool can be achieved using the --remove-home option. If the --home option is given, deluser will only remove the user if the directory given to the --home option matches the user's real home directory.

#deluser --remove-home admin

Groups Administration in Linux

Add New Group in Linux
groupadd -- Create a new group

Syntax
groupadd [-g gid [-o]] group

For more options check groupadd man page

#groupadd test1
This will create a test1 group

addgroup -- add a group to the system

Syntax
addgroup [options] [--gid ID] group
#addgroup

Enter a groupname to add: admin1
 Adding group `admin1' (1001)...
 Done.

Modify Group in Linux
groupmod -- Modify a group

groupmod [-g gid [-o]] [-n group_name ] group

#groupmod test1 test2
This will modify group name test1 to test2

Delete group in Linux

groupdel -- Delete a group

groupdel groupname

#groupdel test2
this will delete the test2 group

delgroup -- remove a group from the system
Syntax

delgroup [options] [--only-if-empty] group

#delgroup --only-if-empty test2
Removing group `test2'...
 done.

groups Command
print the groups a user is in

groups [username]
This simple command displays what groups a user is a member of. It takes the username of user as a parameter. If no username is given, it defaults to the current user.

# groups
root

#groups admin
test1 : test2

File        Purpose
/etc/shadow    Secure user account information  
/etc/passwd    User account information  
/etc/gshadow  Contains the shadowed information for group accounts  
/etc/group       Defines the groups to which users belong  
/etc/sudoers    List of who can run what by sudo  
/home/*           Home directories  

List all:

All users:

$ getent passwd

All groups:

$ getent group

All groups with a specific user:

$ getent group | grep username

 

---

http://linoxide.com/how-tos/linux-groups-gpasswd/

Manage Linux Group Using gpasswd Command

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: