I understand they roll out city-by-city and have a large pre-launch registration base prior to opening the app to that city.
They are getting interest through their traction in other cities. Since they are already in 20 cities and have approximately half a million users if you look at their Android app (https://play.google.com/store/apps/details?id=co.hinge.mobile.android), they can simply wait until one city "tips".
This might also just be a strategy to artificially use scarcity since it works sometimes, such as with Mailbox(http://www.nirandfar.com/2013/07/psychology-of-scarcity.html).
I think it doesn't really matter if they launch city after city, as they have $8M in funding and can just spend $5,000 on Facebook Ads and get 1,000 user in one city within a day. The effect of scarcity makes sense, if you are new and if you're a hyped startup such as Mailbox used to be. On the other hand, it makes sense if you don't have the resources to launch in many cities such as Uber, because they need the drivers first.
I can also schedule a call, where I can tell you more about this as I've done a lot of apps and have a social network myself with Tennis Buddy.