There's actually no really firm answer here.

http://eltj.oxfordjournals.org/cgi/pdf_extract/38/2/138

When in doubt, I think you should go with what sounds more natural to most readers, which would be "Look who's finally getting married." It's true that we know from context a plural number of people are getting married, so if you were to turn it around like Mark does, you'd definitely use "are." But, given that there are a lot of experts who think that "who" as an interrogative pronoun is always singular, you have some latitude here, and should go with the one that's less awkward sounding.