I've had issues with having to forward port 7777 so first login to your router and forward that port (unless you're joining a game, in which case ignore this part)
Then click "Play and Host" (or was it "Host and Play"?) and share the IP with your friends who want to play.
If you're joining one of your friends games, ask them for their IP (Found by clicking and holding the button next to "Host: " on their end) and enter the IP under the button you would use to host a game on the main screen. Don't mess with the port unless they're hosting on a port that isn't 7777. After that, just click join and hope for the best.