The base game is only single player. You have to get the expansion to have multiplayer.

Still, it would be nice to have better descriptions about this. It's barely mentioned in the expansion description.

EDIT:
Nope, this is wrong, it does need a multiplayer tag.

Both Drox Operatives and Invasion of the Ancients are multiplayer games. Kivi's Underworld is the one that have multiplayer only with the expansion.

Interesting, I say that because my brother tried to show me the game by installing it on his laptop and playing with his desktop, but he only put the base game and it had no multiplayer button. He said the button appeared after he installed the expansion.

Maybe we missed something... or something has been patched since then, it was almost a year ago.

That's possible. The latest version of the base game has multiplayer for sure. I played the game long ago but not really cared about that option.

You're right, just bought it so I could check. :) Okay not really, but I did just try the vanilla version and it does indeed have multiplayer.

Lol. Anyway there aren't too much players to play with. I've tried it with a friend one time and that was all the multiplayer experience I have ever seen in Drox Operatives.