The IP of the server is showed on the chat area. The form of an IP is a block of 4 numbers xxx.xxx.xxx.xxx. Here an example when you click on the button Start of the previous dialog. You can push on ALT-TAB to switch applications. When all players receive the IP and the Port, they only need to enter these settings in the address area in the following dialog and click on the button Connect.
Many reasons are possible.
The most frequently problem is that the server didn't give the good IP to other players. Also, you must wait for the server to click on Start before to click on Connect. The Port and the IP must be the same as the server on the client side.