If it seems that players are unable to join your server after starting it from the “Host server” menu, you may have to setup port forwards on your router in order to let incoming connections through. On many router models this can be done by navigating to http://192.168.1.1/ using your web browser, finding the port forwarding settings (sometimes called “application sharing”) and forwarding one of the ports using the following settings:

Service/Application: barotrauma
Port: The port you have selected for your server in the “host server” menu (14242 by default)
Protocol: UDP

The exact steps for forwarding a port depend on your router’s model, but you may be able to find a more detailed port forwarding guide for your particular router on http://www.portforward.com or by googling your router model + port forwarding.