Wow. Just wow. I am beyond speechless at the level of passivity being advocated by so many here. This is a HUGE betrayal by someone you trusted, it is something that you had almost no way of detecting even IF you had reviewed the paper, and while you will never give another person this much trust ever again, you DO NOT deserve to suffer for this in the slightest.
Are you sure it was this girl, and not one of the other researchers, who plagiarized?
I would gather every shred of evidence I could. I would contact IT and ask them to provide me with copies of the emails, offer to pay a fee, flirt with a guy there if I had to, figure out what was needed. I would go over the paper turned in with a fine-tooth comb and have the accusing professor identify the issue (by showing up very sweetly during office hours, explaining the situation and acknowledging that he is in a difficult spot here, but I am curious as to what and where the plagarism occurs in the paper). I would welcome the meeting with the dean. I would march in with my research, my IT-recovered emails, my highlighted copy of the final paper showing what was plagarized, and anything documenting my portion of our research and cross-referencing its presence in the paper. You might be able to show that the section containing your research has nothing plagarized. If nothing else, your proactive attitude will convince most teachers of your story. But at least you stand a good chance at making them sit up and realize that YOU did not plagiarize. If you trust your fellow team members enough to bring them into this, that's all you and might help things a little. But build your own case and present it to the Dean.
When I was a freshman in undergrad (and I was a year younger than everyone else in my class and a little shy at that point), my professor withheld my first paper and said that it had to be plagiarized because no freshman could write at that level. I was furious that he was singling me out, implying that I didn't look smart enough to perform that well (since my looks were the only exposure to me he had to go on), and asked for proof. He had none. I was sitting in his office, my voice shaking with anger, and told him that since he had no grounds to accuse me of this and nothing to compare my performance to, I'd would not accept his biased decision. He acted very concerned about my "attitude" but agreed with withhold a grade until the end of the semester "so he could have something to compare it to." I marched into our first "blue book" exam, grabbed his blank book, sat in the front row directly in front of his desk with both hands flat on the surface and wrote the entire exam, word for word, finished it and marched out before anyone else (I'd really overstudied but he didn't need to know that). He asked me to come by the next week, apologized for the accusations, gave me an A+ on both the test and the paper, and I ended up being his research assistant the next semester.
Sometimes teachers are very very wrong. Sitting back looking sheepish and taking your licks does nothing but penalize you for hard work and endanger your future prospects. Definitely deal with this. These are not infallible parent figures - they're just your average jane and joe working from 9 to 5 and making the same snap judgments we all do. Don't let the typical university professor's position on the lectern intimidate you. They're there because you pay and you show up. And you have the right to demand and receive the level of academic recognition and reward that you worked so hard to earn.
