See this page for more details about user management:
https://help.ubuntu.com/lts/serverguide/user-management.html

Print the user who is currently logged in:
$ users mtitek admin

Use the useradd command to add a new user (man useradd):
Usage: useradd [options] USER Options: -b, --base-dir BASE_DIR base directory for the home directory of the new account (default "/home") -d, --home-dir HOME_DIR home directory of the new account (default: "/{BASE_DIR}/{USER}") -m, --create-home create the user's home directory -G, --groups GROUPS list of supplementary groups of the new account -s, --shell SHELL login shell of the new account (/bin/false to stop SSH logins) -c, --comment COMMENT comment -e, --expiredate EXPIRE_DATE expiration date of the new account -f, --inactive INACTIVE password inactivity period of the new account
The command useradd affects two files "/etc/passwd" and "/etc/group".
Here's how data is structured in these files:

• /etc/passwd
  $ cat /etc/passwd | grep mtitek mtitek:x:1000:1000:mtitek:/home/mtitek:/bin/bash
  Where:
  • mtitek: user name.
  • x: Placeholder for password. The password is obtained from the "/etc/shadow" file.
  • 1000: User ID.
  • 1000: Group ID.
  • mtitek: Comment.
  • /home/mtitek: Home directory.
  • /bin/bash: User shell.
  
  
• /etc/group
  $ cat /etc/group | grep mtitek mtitek:x:1000:mtitek
  Where:
  • mtitek: group name.
  • x: Placeholder for password information. The password is obtained from the "/etc/gshadow" file.
  • 1000: Group ID.
  • mtitek: Commas separated list of users that belong to the group.

Examples:

• Create a user named "mtitek1" with a default group named "mtitek1".
  $ sudo useradd mtitek1 $ cat /etc/passwd | grep mtitek1 mtitek1:x:1003:1004::/home/mtitek1:/bin/sh $ cat /etc/group | grep mtitek1 mtitek1:x:1004:
  
• Create a user named "mtitek1" with a default group named "mtitek1".
  + create the home directory "/home/mtitek1".
  + set the "/bin/bash" as the default shell for the user.
  $ sudo useradd -m -s /bin/bash mtitek1 $ cat /etc/passwd | grep mtitek1 mtitek1:x:1003:1004::/home/mtitek1:/bin/bash $ cat /etc/group | grep mtitek1 mtitek1:x:1004:
  
• Create a user named "mtitek1" with a default group named "mtitek1".
  + create the home directory "/home/mtitek1".
  + assign the user to group "group1" and "group2".
  + set the "/bin/bash" as the default shell for the user.
  $ sudo useradd -m -G group1,group2 -s /bin/bash mtitek1 $ cat /etc/passwd | grep mtitek1 mtitek1:x:1003:1004::/home/mtitek1:/bin/bash $ cat /etc/group | grep mtitek1 mtitek1:x:1004: group1:x:1001:mtitek1 group2:x:1002:mtitek1
  
• Create a user named "mtitek1" with a default group named "mtitek1".
  + set the base home directory to "/mtitek1_home_dir".
  + create the base home directory "/mtitek1_home_dir".
  $ sudo useradd -m -d /mtitek1_home_dir mtitek1 $ cat /etc/passwd | grep mtitek1 mtitek1:x:1003:1004::/mtitek1_home_dir:/bin/sh $ cat /etc/group | grep mtitek1 mtitek1:x:1004: $ ls -al /mtitek1_home_dir drwxr-xr-x 4 mtitek1 mtitek1 4096 Nov 27 12:51 . drwxr-xr-x 26 root root 4096 Nov 27 12:51 .. -rw-r--r-- 2 mtitek1 mtitek1 4096 Nov 27 12:51 .profile -rw-r--r-- 2 mtitek1 mtitek1 4096 Nov 27 12:51 .bashrc -rw-r--r-- 2 mtitek1 mtitek1 4096 Nov 27 12:51 .bash_logout
  
• Create a user named "mtitek1" with a default group named "mtitek1".
  + set the base directory to "/user_base_dir".
  + create the base home directory "/user_base_dir/mtitek1".
  $ sudo mkdir /user_base_dir $ sudo useradd -m -b /user_base_dir mtitek1 $ cat /etc/passwd | grep mtitek1 mtitek1:x:1003:1004::/user_base_dir/mtitek1:/bin/sh $ cat /etc/group | grep mtitek1 mtitek1:x:1004: $ ls -al /user_base_dir drwxr-xr-x 4 root root 4096 Nov 27 12:51 . drwxr-xr-x 26 root root 4096 Nov 27 12:51 .. drwxr-xr-x 2 mtitek1 mtitek1 4096 Nov 27 12:51 mtitek1 $ ls -al /user_base_dir/mtitek1 drwxr-xr-x 4 mtitek1 mtitek1 4096 Nov 27 12:51 . drwxr-xr-x 26 root root 4096 Nov 27 12:51 .. -rw-r--r-- 2 mtitek1 mtitek1 4096 Nov 27 12:51 .profile -rw-r--r-- 2 mtitek1 mtitek1 4096 Nov 27 12:51 .bashrc -rw-r--r-- 2 mtitek1 mtitek1 4096 Nov 27 12:51 .bash_logout

Update the account information of the user (man usermod):
Usage: usermod [options] USER Options: -d, --home HOME_DIR new home directory for the user account -s, --shell SHELL new login shell for the user account -G, --groups GROUPS new list of supplementary GROUPS -a, --append append the user to the supplemental GROUPS mentioned by the -G option without removing him/her from other groups -c, --comment COMMENT comment -L, --lock lock the user account -U, --unlock unlock the user account -e, --expiredate EXPIRE_DATE set account expiration date to EXPIRE_DATE -f, --inactive INACTIVE set password inactive after expiration to INACTIVE
Examples:

• Change the user home directory and the user shell:
  $ sudo usermod --home /home/mtitek1_new_home_directory/ --shell /bin/bash mtitek1
  
• Restrict the user from accessing his account:
  $ sudo usermod --shell /usr/sbin/nologin mtitek1

Delete a user (man userdel):
Usage: userdel [options] USER Options: -r, --remove remove home directory and mail spool -f, --force force removal of files, even if not owned by user
Examples:

• Delete the user "mtitek1":
  $ sudo userdel -r mtitek1 userdel: mtitek1 mail spool (/var/mail/mtitek1) not found userdel: mtitek1 home directory (/home/mtitek1) not found

You need to configure AccountsService.
To hide a user named "admin1", create a file named "/var/lib/AccountsService/users/admin1" and add the following.
$ sudo vi /var/lib/AccountsService/users/admin1 [User] SystemAccount=true

You may need to edit file "/etc/ssh/sshd_config" and do one the following: $ sudo vi /etc/ssh/sshd_config # permit root login: yes/no PermitRootLogin no # allow some users AllowUsers user2 # deny some users DenyUsers user1
Reload SSH configs: $ sudo /etc/init.d/ssh reload