The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
As of right now, there are 2 scenarios of defeat in coop:
Last ACU dies
Any ACU dies
And there is no way to know until your 1st defeat, and then you have to remember for each specific mission. This should either be changed to only 1 of the 2, or something in the game should state which 1 it is.