I think you can't. The modem will allow only one TCP connection at a time, aka one client served. This is the limitation of the integrated TCP stack.
If you need more clients, I'm afraid you have to implement a TCP stack with multiple sockets capability (in an external MCU), all by yourself. It is not an easy task, you have to write all stack starting from PPP, and ending with HTTP.
Another problem would be the allocation for a real IP address. Be really careful, the IP address you see when you attach to GPRS context is not always real. It happened to me when testing with Orange.