A friend of mine when to the convention a couple of weeks ago and was telling me the person she went with was a D23 holder. What is D23? I've looked around and haven't been able to find anything.

http://d23.disney.go.com/

It is a club, it cost $75 a year to be in.

What are the advantages of being a member of this club? Where can a person go to find out more information?

Here is a link to the FAQ on D23 site. Linky (http://d23.disney.go.com/faq.html)
What do I receive with membership?

One year subscription (four issues) to "Disney twenty-three" publication. D23's exquisite collectible magazine is filled with stunning photography, dazzling illustrations and Disney insider perspective. It's an all-access pass to discovering the magic of Disney's past, present and future!
Membership Card and Certificate. The membership card and personalized, suitable-for-framing charter member certificate feature many of Disney's most beloved characters as drawn by Disney Master Artist Dave Pacheco.
Surprise Collectible Gift. Just for D23 members, an exclusive Charter Year collectible gift from the new Walt Disney Archives Collection.
Exclusive Special Events and Merchandise. D23 offers its members unique special-event opportunities throughout the year, as well as access to member-exclusive, limited-edition merchandise and collectibles.
Discounted admission to the D23 Expo (more on the Expo below).
FanFare E-Newsletters. Subscription to bi-weekly D23 FanFare e-newsletter with breaking news and special offers. (You may opt out of receiving FanFare upon receipt of each e-newsletter).

What are the advantages of being a member of this club? Where can a person go to find out more information?

The link that was provided above should pretty much have all the information you're looking for.

Thank you