Asking for help

[Cautious]

I am a long time member of Greekchat, but I do not wish to post under my normal name since there are people on here that go to my school and I wish to keep this as anonamous as possible. I am asking for advice. If anyone knows who I am, or the situation, please PM me. I am taking the last avenue I have abailiable.

Let me start at the beginning of a very strange and long ride:
Last semester, I met a guy, I'll call him Mark. He walked up to me and said he knew me, but I didn't really recognize him. We talked for about 15 min, then I left to meet a friend. When I got home, there was a message on my answering machine saying "hi, this is Mark, I hope you don't mind but I looked up your number..." noramally, it wouldn't bother me, but I don't remember ever giving him my last name. I decided that this could either be sweet that he took the time, or creepy, and I erred on the side of creepy and did not call him.
Fast forward to this semester. I saw him at the bar, but didn't remember where I knew him from. He came up, re-introduced himself, but he seemed like a nice guy, so we talked for a while at the bar, and then back at his place. He asked me if he could call me sometime, and I said sure, even though it was Spring Break the next week and it was one of those casual one time things. I didn't think that he would actually call. When I got back from break, we talked a couple of times and I became less and less interested int alking to him. I then found out a sister had also hooked up with him in the past week. He had used the same lines on her and said the exact same things, which was a little weird. He had also asked if he could call her, to which she said yes, what was the harm. Later she decided this had been a one night thing and didn't really want to talk to him. He tried to call her, but she said that she didn't really want to talk, and she thought that was the end of that.
That weekend, he called me from another City and left a message on my machine asking if I knew anything about the rumors going around about him, and that they had reached a potential employer. I had no idea about anything that was going on, and asking him what he was talking about when he finally reached me. He said that women in my house were saying that he was a stalker and that somehow the people that had been having an interview with that weekend had heard.
This was the first I had heard anything about rumors and I had no idea why a business would have heard them in the first place, both of which I told him. He tried to call my sister when he got home to confront her (why, I still don't know) and asked her if she knew how the rumors had started. She said she didn't know what he was talking about and really didn't want to talk to him anymore. This is the point that he came to me. My sister had just called me saying how weird the conversation had been when he knocked on my door. He confronted me, asking where the rumors had started, and I said that him telling me was the first time I head heard about it. He continued to try to ask me what my sister knew about it saying "she said she didn't know how they started, but when I asked her who said it, she said she didn't know. I know that means she does, and why won't she tell me?" At this point I grew extremely uncomfortable with the situation, and told him that I didn't know what was going on, I didn't know who started the rumor and he should just go to the source if he knew who that was. He said he didn't and thought I would know, which I thought was weird. I told him again that I was uncomfertable that he was confronting me about a subject I obviously knew nothing about and I didn't want him to call me or stop by again. He kept talking, and I repeated myself and he finally left.
Recently a friend of mine joined his fraternity, XYZ, not knoing any of this. Apparently it has grown to be quite an issue within the house, and Mark is already not supposed to go near another house because apparently there was an issue there and she is uncomfertable with him as well. My friend called me aand said that he knew is involved members of my house and did I know any of the story. I went and had a long conversation with him and some of his brothers last night, eventually revealing that I had been one of the women involved. The issue with Mark has become such that women are unwilling to participate in activities with XYZ because they feel so uncomfortable with him. Because of one man's repeated actions, the whole house is getting a bad reputation. (side note: they are recolonizing this year, so this can really be a killing blow to a brand new rep.) Now I have been asked to either sit down with key members of XYZ and explain my side or write a written statement, unsigned of course, to remain anonamous. It is up to me if I do either, both or neither, but I do not believe this man is upholding the ideal of his chapter, and he will be their downfall. But this is not for me to decide. He has told them that they have no evidence so they shouldn't kick him out, and I am their evidence now if I so choose.
My dilema: I would love to be able to help out and tell my story, which would be anonamously. I am afraid that there will be retaliation, either against me or my house by Mark if I do. I know the men I talked to last night are behind me, and after hearing my side of the story, they are more determined than ever that the truth be heard.
One more kink in the line: At one point, I'm pretty sure he told me that he had pledged and initiatied AB chapter but after initiation, decided that the guys were not waht he thought, go out, and became a founding father of XYZ instead. I know this is not allowed by IFC, and I told my friend that I wasn't sure, but that they should look into it just in case.
I know it sounds like I just want to ruin this guys, but that is not my intent. My intent is to do what I can to help out a chapter of friends, knowing it may mean telling a personal story. What do I do now? Do I meet with them, write them a letter, or let them just take my tesimony back to the chapter as heresay? PM me if need be, or if I haven't been clear. I'm sorry this was so long, but this is happening THURS and I am at a loss.