Valid keys are required for joining multiplayer games, it does not matter if some consider it "completely boring" such opinions are useless and irrelevant.
Multiplayer is not only a part of the game, it is also an advertised part of the game, so expecting the keys to be provided is hardly surprising. Unless keys that allow access to multiplayer are provided it is false advertising and deceptive, which would be uncharacteristic for GOG.
I've had to contact GOG support before for multiplayer keys to games from GOG and received them without trouble, it may not be the best way to do it (or even good or efficient) but its been reliable so far.
We are working on getting the CD keys for multiplayer
and as soon as we get everything straightened out, you will see the key on your game shelf when you expand the box for the game and, thus, you won't have to contact Support for one. Sorry about the delay with the keys but rest assured we are doing everything possible to get them to you in a timely fashion.