Many races are being announced on the Discord server in the #lobbies channel. If someone announced a race, copy their IP (CTRL C) and start up the game.
If the host didn't post any IP, you can always ask for it.
Then select Start Race and Multi-Player. Select Join Game. You will be prompted for the Host Computer. Now paste the IP addres (CTRL V) and press enter.
You will then join the lobby.