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!
I checked the map pool in the client, there's still a small mistake : it has Adaptive Corona, instead of Land Wilderness (according to Ftx thread). Not very important but since I noticed I thought i should notify it here.
You can start playing TMM now! Download link is available in this post:
https://forum.faforever.com/topic/854/team-matchmaker-beta-release-now-available