I've never developed for iOS, but on my iPhone when I go to okcupid.com it somehow 'redirects' to the okcupid app on my phone. So it seems there is some capability for this built in that website developers can take advantage of.
There is: Apps can register to handle certain URL schemes, so all you need to do is put in a redirect to (for example) "okcupid://profile/109" and, since Safari doesn't handle URLs with the okcupid scheme, the OKCupid app will be asked to open the URL instead. But again, this would require the plus.google.com team to actually implement it.
It looks like they basically ask whether you want to use the site or the app the first time you visit on a device, and if you choose the app, it sends you to the App Store and sets a localstorage setting that triggers a redirect on subsequent visits. (Edit: OK, it's a little bit more sophisticated than that. But I think that's not too far off.)