Have never been to a DIS or MouseMeet, or anything like that, but used to go to big get-togethers (around 150 or more at main event, sometimes around 40 or so for events, especially meals during the week (try wrapping your head around making ADR's and details for 40 - was NOT easy!!) usually annually many years ago, did it approx. 10 times. Was always a little scary, but I can also espouse what everybody has said here - you always "know" peeps from the boards, and as time goes on, you "know" more and more peeps on the boards, and in "real life". The nervousness goes away very quickly, especially if you remember you always have the love of Disney in common, so start and go back to that if unsure. And in time, those peeps can - and possibly will - turn into lifelong friends. I have current, close friends that I met through my love of Disney (and going to the get-togethers, which we called Gatherings, as in Gatherings of Disney Friends) that I met on Prodigy (an online service, WAAAAY back when!) and then in WDW back in '92!!! And they are a treasured part of my life!!
EDIT: Remembered a saying I saw/herd I don't know where that all this makes me think of: Strangers (you can insert DIS'ers here) are friends you just haven't met yet!