Start up a game with a password and crossplay turned on for it. Then once you load in it will show you an GAME CODE for inviting a friend. If you didn't see the code when it first popped up, just press START on controller or ESC on keyboard and at the top you will see a game/friend code listed. Give that code to your friends you want to join.

I did it up earlier today, started a game on PC with crossplay on and then used the code on my xbox to join up. We had one person on xbox and one on my pc/steam, seemed to work great. You might have to wait a little while to join after the person has started the server. For the person joining just enter that code.