I am one of those IT rarities - a female. And from my experience, this is my attempt to answer the question.
If you are a woman on the outside of the clique you are there for one of two reasons: Either these are men that don't believe a women can't perform in this field or they simply don't know how to respond to women, seeing as they are so frequently (sarcasm) working with them. If you are on the outside for the first reason, chances are you won't win them over at all, but if you do, it will be by showing them you know what you are doing and that you are not a bumbling, ditzy 'girl' hired to fill a quota. In this day and age though, I would hope it is the latter. Don't try to join the clique just to join the clique; do it because you have common interests and could actually have conversations with these guys on the same level. If you are trying to get in, maybe you should approach the group about doing lunch. If you feel isolated, then try to do something about it. If it fails, well at least you tried. You can't rely on them to approach you.
To the men - if she is an IT 'girl,' chances are she is not like most women anyways. I have actually found that I share more opinions with men than I do women. I myself tend to see many women as annoying, and ditzy and overtly obsessed with shopping and with whom I have nothing in common. However, the women I have met in my field, tend to be different. They are less petty and easier to talk with and who actually know their s***. If you get any impression she is like me, she should be easily approachable and easy to include in the group. Despite many previous posts to the contrary, chances are she wants to be treated as "one of the guys". And you may be suprised to find out, that its not that hard to do.
On the other hand though, being women in a male-dominated field does put some on the defensive (as the lawsuits would imply). I would have to agree with most of the previous posts about approaching her as a group or sending out the e-mail to do lunch. Lunch is a great ice breaker. If I was worried about being hit on, I would feel less intimidated by this approach. And to women in this position - calm down. Not every guy who approaches you is trying to get in your pants. Even though I am sure the thought may have crossed their mind at least once.
People on both sides of this gender thing just need to lighten up. Learn to relax. Thats probably a key thing to trying to include yourself/'the girl' in the group.