Explain it to her, then have her sit down with you and show her the board a bit. Show her some of the more sane and intellectual threads. :o)
Just tell her. It’s a part of your life, isn’t it ?
My husband wasn’t even slightly interested in that sort of thing and just told me to go and have fun. Lately he’s started becoming more interested and has even admitted he’d like to turn up next time to meet some of the people I talk about so often 
My first dope fest was just last month in NYC. It was my wifes idea, so I guess that made me the drag allong. I wasnt against it, I just never went to this particular board before. So I went, met a whole slew of cool people , like Gorgon Heap who only lives like 10 mins from me. Your GF should dig it, especialy if the Charlotte dopers get all drunk and silly like the NY dopers.
Definitely invite her! I never had this problem with my SO (whom I met at a Dopefest), but some of my non-Doper friends were initially pretty skeptical about my online friends. The two groups have successfully merged, though, and I’ll often go out with a group that comprises Dopers and non-Dopers.
Wait, wait.
You guys date people who don’t frequent message boards and leave at least three IM programs online at all time?
You can have sex with someone who doesn’t know what IRC is?
You can honestly look someone in the eye that has NO idea what goatse is or why all their base are belong to us or why they should bring pie when they come back and tell them that you love them?
Holy god, and I’m the freak?!