Privileged port
 

I want to allow only one user on the system, do host a service on one port (port <1024)..
how can i do that?

Privileged port binding is for root only. Some apps do run as lesser-privileged user later on, but they're started by the root account user and drop privileges right after. Allowing unprivileged users (that is with an ID over 500 on regular systems) to run daemons on privileged ports is not advisable.
So, if you could be a wee bit more verbose, like what user, what application and most importantly: why?