I would suggest talking to some of the admins in your company, any of your friends that are admins, etc.  This does 2 things, starts you networking already and let's you know what organizations they are aware of and how they feel about them.  I personally don't belong to any, but just from reading various posts here I can tell that an organization that is great in one place might not have the dedicated people/same effect in another.  If you get a chance, Skillcraft offers some Admin courses (a day long seminar where lots of topics are covered) and are perfect ways to network with other admins from various fields.  If you go to one of these, or any other setting where you will have a chance to talk with other admins, be sure to step outside your comfort zone and talk with as many folks as you can, get a group together for lunch and breaks, and make some friends.



Good luck!