When my first transplant surgery was cancelled because I had antibodies against my donor (my sister), my doctor first suggested I try to find a new donor. When the word got out, I had several people ask about the requirements for donating and indicated an interest in looking into it. What I would do is e-mail them the criteria and my transplant coordinator's phone number for further info. I would also stress that it was a big decision and something they should discuss with their immediate family. I would also ask that they NOT let me know if they decided to get tested, as I didn't want to a) have them feel any pressure from me or anyone else (especially non-related potential donors) and b) be disappointed if it didn't work out.
I did have a couple of friends tell me afterwards that they had been tested (and were not a match), which was fine to know at that point, since it was after the fact. I was so moved and honored that they would even consider doing such a thing as donate a kidney to me, and I would tell them! They would usually say that they were surprisingly disappointed that it didn't work out, which said to me that they were really serious about going through with it.
As it turns out, my sister was able to donate a kidney to me through a positive crossmatch program at Mayo Clinic on December 5, 2006! I have since heard from other friends (and even acquaintences!) that they were tested as well. I can't tell you what that means to me...it brings tears to my eyes that they care so much!