IIRC this is not possible, or if it is it is not recommended for security reasons.
But if you want users to be able to listen on a specific port you could always use a TCP forwarder like
rinetd or iptables rules to redirect connections to that port to one they can listen on and have them set their service to listen on that higher port.
For instance the line
aa.bb.cc.dd 80 127.0.0.1 8000
in rinetd's configuration would forward connections to port 80 on address
aa.bb.cc.dd to localhost port 8000 which a non privileged user can listen on. An equivalent iptables rule would be something like
/sbin/iptables -t nat -A PREROUTING -p tcp -d aa.bb.cc.dd --dport 80 -j DNAT --to 127.0.0.1:8000
Either way gives you much more fine grained control than letting any user listen on any port.
The iptables approach has the advantage that the listening application will see the IP address of the calling client (with the rinetd method it would see all connections as coming from the local host). The iptables method would also allow for UDP as well as TCP.