The following commands will need to be prefaced with the sudo command. Please see RootSudo for information on using sudo.
The addgroup command is used to create a new group on the system. To create a new group, type:
addgroup newgroup
The above command will create a new group called newgroup.
The adduser is used to create new users on the system. To create a new user, type:
adduser newuser
The above command will create a new user called newuser.
To assign a password for the new user use the passwd command:
passwd newuser
Finally, to assign the new user to the new group, type:
adduser newuser newgroup