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!
This is already part of the FAQ : https://forum.faforever.com/topic/252/why-do-i-need-to-link-my-account-to-steam
https://forum.faforever.com/topic/281/i-bought-forged-alliance-but-not-on-steam-can-i-still-play-on-faf
