Co-op can be either LAN or online; there are no public servers/matchmaking. For co-op, the person who joins a game basically takes over one of the characters that the host would otherwise be controlling themselves. They can then run around together, or separate and get into combat alone (which would be much harder, of course) while the other player is solving quests or talking to people, or even getting in to a fight of their own.
Totally noob question, if there are no servers or matchmaking, how do you meet other players online?
Well, that makes two noobs, then. :-)
I suppose one has to count on friends / acquaintances, or possibly one might post here or in Larian fora once the game is out in case one's friends are not into gaming, and one wants to give the co-op mode a try.
I for instance only have one person amongst my friends who is into gaming at all, but mainly on consoles.
However, I must try to exert beneficial influence as having seen the game-play demo, I totally want to co-op up with him because he has totally opposite take on character build up, and he enjoys going evil whereas I totally don't.
The idea of re-enacting Beyond Divinity of sorts just seems like a lot of fun to be had, as we'd be dueling on the approach to questing for sure and we do like friendly bickering well enough in real life. Well, only if I will be bullied into evil acts every time I shall have to make an exception to this co-op thing - but righteousness shall surely prevail, lol - at least from time to time.