No, you don't need to bring anything special or dress up or anything.

I'm going over to Professor Sinistra's -- well, so, I keep in touch with Professor Dolohov because he got me this internship and he lends me books. And he wanted me to visit her because he thought it would cheer her up, so he suggested I ask her for help with German. (Healer Korotkova, who I have my internship with, wants me to read things in German. Also Russian and today there was something I'm not even sure what language it was because I didn't recognise the alphabet, I think I may ask Professor Dolohov in case it's Arabic? but anyway.) And then he sent her a message to let her know about the German and she sent me a message and that's why I'm going over there (and I realised I said tea but I meant lunch, I'm going over at noon).

And she asked me to bring Pansy so why not.