useradd -- Create a new user or update default new user information
adduser -- User Friendly Frontend for useradd command
usermod -- Modify a user account
userdel -- Delete a user account and related files
deluser -- remove a user from the system

Groups Administration in Linux
groupadd -- Create a new group
addgroup -- add a group to the system
groupmod -- Modify a group
groupdel -- Delete a group
delgroup -- remove a group from the system
groups -- print the groups a user is in

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Resources
debian admin - Users and Groups Administration in Linux