Legal or not legal?

A friend of mine just made an account at the site I play at because I recommended it to him. Are we allowed to play in the same tournaments together or do we have to make a private table?


Edit: We obviously wouldn't be cheating but we are friends in real life who play together every weekend. He registered so we could play on weekdays as well. Still illegal?
I don't want to get banned if they look at my account and see that I have played like 10 tournaments with the same person so before I do it I want to check.