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When it's time to get your friend, significant other, or that guy you just sort of know into MMO's, don't start in the deep end of the pool.
Choose the game carefully based on complexity and their interests. Even if they are single-player savvy, don't assume that they understand that gameplay continues without them. Don't hold their hand so much that they don't learn things on their own, and don't introduce them to PvP too early. Follow those steps, and you'll have a new in-game buddy in no time.