By default, your friends can join your server over the Internet using its Server Code in the Connect menu without port forwarding <doc_servers_port_forward>.
The downside of Server Codes is they are incompatible with the pre-joining server info screen. The info screen uses Steam's A2S protocol, which can only be queried by IP, so joining by Server Code enters the server immediately. You can also enable Fake IP<doc_servers_fake_ip> to work around this limitation.
After startup, you can run the CopyServerCode command from the server console to copy the code to your clipboard.
Connecting by Server Code utilizes Steam Datagram Relay (SDR). To quote that page: "Relaying the traffic protects your servers and players from DoS attack, because IP addresses are never revealed. All traffic you receive is authenticated, encrypted, and rate-limited. Furthermore, for a surprisingly high number of players, we can also find a faster route through our network, which actually improves player ping times."
Note
Without a Login Token <doc_servers_gslt> the Server Code will change each time your server restarts.