The beta features "all six multiplayer races" which includes the new Imperial Guard, with all new units too. The tests will run through the end of this month.
"Featuring all six multiplayer races including the new Imperial Guard faction, the beta will run through the end of February to test the new Steamworks network and matchmaking features and help balance the multiplayer mode," say THQ.
"All new units for each race will be available in the beta along with the new multiplayer maps that will ship with Retribution," they added. The beta for Retribution is going to run until the clock strikes midnight on February 24th.
You can register for a shotover on the community website. Warhammer 40,000: Dawn of War II - Retribution releases on PC March 4th in Europe.