I have been meaning to do this for a while, and a comment yesterday reminded me. Many games give you bonuses for inviting friends and such, mostly if you are the source for their initial subscription. Feel free to use the comments from this thread to toss up your referral links, “e-mail me for a code,” codes, etc., whatever is relevant to your game. I suggest putting the name of the game in all caps at the top, then saying what they need to do to get the referral, then listing what you and they get from it. “What they get from it” is a useful incentive if they get more than just signing up on their own. City of Heroes, for example, has “welcome back” codes (account inactive for 90+ days) that gives you and your friend 15 days each when they re-subscribe. This is good for everyone, except perhaps the company involved.
Example: WIZARD101, a kid-friendly MMO that plays a lot like Yu-Gi-Oh or Pokémon. Friend code: 49090-08124-81429-46863. If you use my code and subscribe or buy $10 in crowns, we each get 1250 crowns. You might also get a pet or something; the page equivocates.
If you would like to have a post removed (unsubscribed, ran out of codes, etc.), comment again or e-mail me, and I can delete old comments. We may have multiple, competing codes/links for the same game: whee! If the game in question is one of those browser-based things like “have all your friends click this link to be eaten by zombies” or MiniCities or such, please use this post. Please be patient if the comment does not appear immediately, as I will need to check the spam filter before and after work.