From
pokerstars FAQ:
http://www.pokerstars.com/poker/room/faq/
Q. Can I play at the same table as my friend?
A. Yes. PokerStars is happy for friends, relatives, and acquaintances to play at the same table as each other. However, when you do so, you are expected to play just as competitively against your friend as you would against any other player, and you must not share any information about the cards that you hold with them or make any playing agreements in secret (doing so is collusion, which is strictly prohibited).
If you have an agreement to share a
bankroll, stake, or split profits with your friend, then in the interest of game integrity you must not play at the same cash game table or in the same Sit & Go tournament.
Players who live in the same household, whatever their relationship, must not play at the same cash game table or in the same Sit & Go tournament. In some cases, this restriction will be imposed automatically by the game software.