Check the network for any messages sent to this client. If none are found it returns 0. If a message is found it returns a message ID you can use to read the message. You must delete the message using DeleteNetworkMessage when you are done reading it. You should keep calling GetNetworkMessage until it returns 0 to make sure you have received all the messages. You can use the message reading commands to find out which client ID sent the message.
integer GetNetworkMessage( iNetID )