Well I'm not entirely sure I understand your question, but do you mean if say a server listened for connections on one port and multiple clients could try to connect?
In that case yes, each client is going to try to connect to the server on the same port, but once the connection is established the server actually "moves" that connection to a different port for each client. This is all transparent and doesn't require any special coding, I believe they're called virtual ports or something like that.
But think of a web server, every client connects to it on port 80 but that doesn't mean that if, say 10 people are connected to it at the same time they are all being funneled through the same port 80. They just connect on port 80, once their connected they get their own port specifically for the communication between that particular client and the server.
So long as you have a way to take care of all the clients simultaneously (multithreading usually) then multiple clients can Connect through the same port - but the data flow won't occur over just that one single port.