useradd -g USERS_MAIN_GROUP -G SECONDARY_GROUP1,SECONDARY_GROUP2 -m -d /home/USERNAME USERNAME

groupadd GROUPNAME

usermod -a -G GROUPNAME USERNAME

Run as root

passwd USERNAME

Or run as yourself to change your own password

passwd

Change password without interaction (e.g. withing a script)

echo "username:new_password" | chpasswd

passwd -l USERNAME

passwd -u USERNAME

Preferred way of doing it as you don't even get the option of trying to log in.

passwd -d USERNAME

Display max number of processes, etc for a particular user.

ulimit -a USERNAME

Set max number of processes for a user for just that session

ulimit -Su USERNAME