I think it's considered bad practice to define your own URI scheme for your app, though it is very common on iOS since you have no other choice. I thought using HTTP links would be nice since (a) they are much more likely to be linkified when pasted in emails and on websites, and (b) clicking them from non-Android devices will take you to the website where you can learn about it.
You could also have a context header on top saying something like "Join [User] on Nightweb" or "Hey, [User] is on Nightweb!" and then your website follows explaining the concept.
