I don't believe that's a standard thing. You'd probably have to install an extension for whichever flavor of unix you're using. I know there was a network quota module for linux, but that limited bandwidth, not specific ports.
One thing you might be able to do, if your firewall setup allows it, is to tag packets from processes owned by particular users so the firewall can allow or deny them. I think OpenBSD's pf allows this, but I've never looked into it (nobody logs into the firewall itself, and "user" is meaningless outside the machine).
AFAIK it Just Doesn't Work That Way by default on unix in general. There are some flavors with exotic extensions to their networking (like AIX iirc) that allow you to turn on stuff like that if you're on a compatible network…but in general the default attitude of unix is to let users use whatever unprivileged ports don't happen to be in use.
If you're on a unix version that uses iptables, that might be a good starting point for googling an answer. For example:
I was hoping this was as simple as following the "everything is a file" approach and I'd find some magic device, like a /dev/eth0/ip/tcp/4000 or something which I could then assign group permissions on.