Sometimes the difference between staying in a game and leaving it is the company you keep. Even more so than in Second Life, where if you don't find a friend within the first 3 days you're likely not to return, joining or creating a guild is arguably the most important step in developing a lasting relationship with the game, and getting access to all the content a game has to offer. Sure, you can solo for a long time, probably even to 'end game', but you'd be missing out on all the opportunities that only running with a pack can grant.
For those of you who run a guild, how do you choose who gets to join? Recommendation by a current member? Extended invitation to a particularly funny/knowledgeable forum poster? Do you put the initiate through tests? How does the process work?