You can email the person you talked to through Georgetown's website if you know it's a Georgetown employee or student. They won't give you an explicit address unless you go to Georgetown or you at least know someone who does go there right now who can see it internally, and it appears that student and faculty email ids are not of the typical variety, one of my friends who goes to Georgetown for another program has her initials and then numbers.
If worse comes to worse, email the MBA program directly and say I'd like to thank John Smith. I'm sure the adcom will note that you said thank you afterwards.