Well I read this on reditt, go and give it a try
Download the mod [this will change soon as closed beta becomes open]
have an login with https://oreo.skyrim-together.com/ [this url may change]
still in skyrim-together.com, link your patreon [this may change after closed beta]
Still in skyrim-together.com, click on FRIENDS, and add your friend. they must accept on their side.
install HARBOUR. Run, and login with the details you used at skyrim-together.com
To Launch the game:...
5. ensure STEAM is running. 6. Launch Harbour, and click PLAY. 7. When the game loads you will see the lovely ST dual dragonborn logo. Start a new game, or continue a saved game. 8. IN GAME press the Right-Ctrl key. This displays the ST Mod interface.
9. Click on FRIENDS. Next to their name click on INVITE. Your friend will need to accept this invite by again pressing r-ctrl, friends, and then on ACCEPT. If unlike me, you have more than one friend, you may continue doing this for your other friends.
9. The player who sent the invites will have, at the top left of the screen, a list of team members who accepted the invite, and a button marked: LAUNCH PARTY. Other players will not see this button. The "Auto Hide" tick box in this location simply hides the interface when it thinks you no longer need it, it's not a part of the game.
And that's it. Now you giggle and make real those dreams from years ago: wouldn't it be great if we could play together? Thank you to all of the ST Mod team who have worked tirelessly on this.
There is ANOTHER post on issues and fixes, please go there to view fixes instead of replying here, this will keep them all in one place.