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