So, has any backer already got this working? Which ports do we have to forward on the hosting machine? How does direct IP joining work, because when I select "New"->"Multiplayer" I get something along the lines of singleplayer, which means I play with an AI. How's that all work if I want to get my buddy joining over teh internets?

I have not tried, but check your multiplayer settings in the Game section of the options; there are separate options to enable visibility online (Steam, and eventually Galaxy I assume) LAN and Direct Connections.
In theory, as long as you have Direct Connections enabled, you just have to start a multiplayer game, and the other person click on Join from the main menu and use either your IP address or Server ID to join (click on the Direct Connection button).

I don't see a way to get the Server ID on the character creation screen, though (so the other player would be able to customize their own character). Try using your IP address, and only worry about port forwarding if that doesn't work. Alternately, your friend can tell you what build they want for you to create.

Once you are in the game, you can open the Connection Menu (diamond icon top of mini-map) to change your options, as well as get your Server ID (with a button to copy it to the clipboard). This ID changes if you exit to the main menu and start a new game, or exit the game entirely and restart.

Thank you!
Hmm.. the "Server ID" is a nice idea, but it would be no "Direct Connect" if we needed some mediator, would it?
I'm trying the IP when my buddy and I come around to playing this, and since I'm behind a NAT like everybody else, I have to forward something on my router, so I compared the output of "netstat -an" with and without the game and guessing it's UDP 23253 . I've done some research (http://steamcommunity.com/app/230230/discussions/0/522730701726659011/) and there is also 23243.
So, in short:
Server:
1) On your DSL Modem, DNAT UDP 23253 and 23243 to the LAN IP of your gaming rig.
2) Enable "Options"->"Game"->"Direct IP Connect".
3) Use http://heise.de/ip or such to get your ip and tell it your friend.
Client:
Use IP to connect.

Thanks again for getting me on the right path!

Actually, I'd just try the IP address, and check the router/modem only if NAT punch through fails.

From another topic somewhere:
The game's client will try to assign a port in the range 23243-23252, the server in the range 23253-23262. While forwarding these ports on your gateway might help, there is no guarantee the gateway will assign an external port in this range when connecting to our NATPunch server (this is the port that will be assumed to be the external port for all connections).

I've disabled the whole UPNP shebang on my dialup device because that was some time in the headlines as being insecure, so I have to do it the manual way :-)
But you are right, this should not be necessary if you leave the box of your provider as it came nowadays.

Still not had a session because my partner has to study and I've started Divinity 2 already :-)

I'm glad we have something for the future

UPDATE: Works good, but a little stuttery on my partner, we'll try the connect in reverse next time to nail down the issue.

Hello you guys,

Could someone explain to me in normals terms how someone would be able to IP connect to my multiplayer character creation?

Direct connect with the Server ID works, but not during character creation.

First, you would need your IP address (that should be listed by your router, or you can just check a site like http://myip.ms/ ), which you would give to the other person.

Next, start a multiplayer game and wait at the character creation screen. The other player would then need to click on Join from the main menu, in the LAN section click on Direct Connection, then type or paste the IP address (the same as they would for the Server ID).

If that doesn't work, port forwarding in your router may help. Copied from a forum topic: If you have, or are willing to try, VPN software like Hamachi or Tunngle, you could try using that to create a LAN game, then save and for subsequent sessions just use the Direct Connection with Server ID.

Thanks for the advice! We're already using Hamachi, because LAN is Local Area Network and we're not on the same network so I figured we had to use something like Hamachi to bridge the networks, or something like that. I really have no idea exactly but it works with server ID. The Internet tab at Join doesn't seem to be functioning for Divinity: Original Sin, yet. I will try to see if I can make it work with IP at LAN, too!

If the VPN is working with the game, the person joining should see a game listed in the top pane of the LAN section of the Join window. They shouldn't need the IP address or the Direct Connect button. If you do use Direct Connect, you shouldn't need Hamachi.

There are no games listed in the join window, ever. I'm starting to regret getting the game code for GOG rather than Steam :P