I've been to MouseFest the last three years running.
There's no such thing as a "member" web site for MouseFest - there are web sites that sponsor MF, and sites that officially participate, but MF is open to anyone, whether you're part of a web site or not.
MF is a buffet-style event - there are dozens of meets and functions, most of which you don't even have to sign up for in advance, and you can attend as many or as few of them as you want. Some are held just for members of a particular web site (there are usually some DIS meets), but most events are open to anybody. There are a few events that cost money (such as Illuminations and Wishes cruises, Segway tours, etc.), and there are a few that involve meals (so you have to sign up in order for the organizers to make ADRs).
The MegaMouseMeet is held on Saturday each year in a large convention hall at the Swan and Dolphin. The MegaMeet reminds me very much of a trade show, with each of the sponsoring and participating web sites, guidebooks, and other businesses setting up tables. The primary purpose of the MegaMeet is to allow attendees to meet the webmasters (like Deb Wills and Jen and Dave Marx) and authors (like Lou Mongello and Bob Sehlinger) in person, but it is also a great social occasion for meeting other attendees, both from your own web community and from the others. Each sponsor also donates door prizes, such as books, t-shirts, etc., which are given away at some point during the Meet.
MouseFest is a lot of fun for all concerned. I've been scrimping and saving all year to be able to afford to go again this year.