ANSWER:

check_procs command accepts a “-u USER” argument to list the processes for the specified user.

You have to edit you command.cfg (the place where your nagios command are defined), and add something like this:

define command
    command_name    check_user_procs
    command_line    /usr/lib/nagios/plugins/check_procs -w '$ARG1$' -c '$ARG2$' -u '$ARG3$'
    

and create a service check for the user:

define service
    use                             generic-service
    host_name                       YOUR-HOST-NAME   
    service_description             Total Processes
    check_command                   check_user_procs!250!400!USER_NAME_OR_ID
    

you have to replace USER_NAME_OR_ID withe the user info you are checking for.

The first two number specify the limits for warning and critical events.

by Goran Jurić from http://serverfault.com/questions/381940