When I went through Rush I knew the affiliation of one my Rho Chi's (or whatever they're called now) because she was actually one of the reasons I chose to go through rush. I knew her and a few other sisters from summer school BEFORE the disafilliation date (it was toward the end of summer). I didn't say a word to anyone in my group; it was understood that no one was supposed to know.
It was awkward when, on Bid Day, I didn't put her group as my top choice. I was so worried about what to do...but then I was like, I'm not joining a sorority for her, I'm joining for me. I felt so guilty when she brought me my bid and it was my top choice, but not her group.
She was kind of giving me a nasty look. She never spoke to me again after that either. Oh well!
ETA: I have always thought that you can SO tell which group they belong to. On the last day of rush our Rho Chi's had us all list the name of the Rho Chi and what group we thought they were in...I know I got them all right, as I'm sure most everyone else did as well.