Hosting and joining
There are three connection types available:
Lets you host the game on your local network (usually your home).
You can play with your Steam friends using the Steam integration.
Steam connections need to be accepted by the host in the chat window:
Note that host's Steam profile can't be private.
(LAN or Steam is recommended if you just want to play with your friends)
You need to get your public IP address and do port forwarding (which is router specific, but many tutorials are available on the Internet). The mod binds and connects to port 30502 by default. The protocol is UDP. Make sure a firewall isn't blocking your connection (especially if you are on Windows).
Connecting using a VPN service like Hamachi is also possible. For Hamachi, the host needs to direct host on their IPv4 address, then players should connect to that IP. No further configuration should be required.
Some users have reported that disabling your antivirus can help with connection issues.