When I enable additional users, there isn't any way to invite them, and the Unlink All Users button doesn't do anything. Any ideas?
Once you've enabled additional users, you can just give your app to them for testing. You don't have to invite them in particular... the first 100 users to authorize your app will succeed.
I feel I must be missing something. How do I "give" them the app?
It depends on what kind of app it is. If it's a web app, tell them the URL. If it's a desktop app, send them a copy of it. If it's a mobile app, install it on their phone.
There doesn't seem to be a way to reply or ask a follow up. For a web app, what is the app's url? I seem to be missing something here.
I just mean the address of your app, which is up to you and wherever you're hosting your app. (E.g., the address for Amazon is http://www.amazon.com.) It has nothing to do with the fact that your app has a Dropbox integration.