Normally I don't comment on Show HNs, but there are a lot of good things to say about what you made. It was a great idea to make this SMS based, as it is a universal medium and most people aren't willing to download Yet Another App. Also if I am planning a gathering, it's more likely that I'll have everyone's phone number, and much less likely that I'll know all of their email addresses or even be friends with everyone on Facebook. The part I appreciate the most is that you don't need the app to RSVP. Most developers would text a "download the hobnob app to RSVP" link to people, but clearly this is far superior since it makes it super easy to respond. The text message picture is beautiful.

I honestly don't have anything negative to say, the app is beautiful and the entire experience is quite polished. Really nice work!