The Groog were a humanoid alien race.
At first it seemed as if the Groog were attacking Mary and Bill. But it turned out that they tried to help them fighting some enemies that they couldn't see. The enemies were the Eleventh Doctor and Amy Pond who were in another dimension and tried to contact Rory Williams, Bill and Mary. Amy and the Doctor thought the Groog were the enemies.
After Amy and the Doctor returned from their dimension, Bill and Mary invited Amy, Rory, the Doctor, Lan and his friend to breakfast and everyone accepted the invitation. The space ship of the Groog only needed a few repairs, and the Groog would soon be able to use it again. (AUDIO: The Empty House)