Graffiti Shirt
 

I don't think you should have a t-shirt. Usually when we have graffiti parties you simply bring a blank white shirt, and that's your souvenier (although I would never wear any of my graffiti shirts out in public).

If your creating a t-shirt that people will get after the party, then perhaps something like a brick wall with "spray painted" writing of the details of the party (whatever you normally throw on there - date, chapters, university, party name).

what happens on our campus is what they call highliter parties. you go to the party with a white shirt and everyone writes on it in highliter. then you have a souviner of the fun that you had.