I started reading the forum around the same time as the Boston gathering that Viva mentioned, so I also went throughout the same process of wondering what the heck these gatherings are all about!
Although I've never been to one, I feel these gatherings are such a special part of YLF. Just a really neat example of how this really is a welcoming and global community of friends. You'll occasionally see folks suggest a "meet up" in the general or off-topic section of the forum. It can be a simple as, "I'm going to be in X city on X dates, does anyone want to get together for shopping and lunch." They are open to everyone. I think there have been discussions about meet ups in Las Vegas, Michigan, and California this summer/fall in addition to the BIG ONE in Seattle. Seattle is exciting because both Inge and Angie will be there. Wish I could go!
Gigi linked to the threads - there really isn't a more official source of information. Everything just evolves on the tread as a conversation like everything else around here!