I have multiple accounts on my Android device. How do I unlock Join with a different account?

Make sure that you install Join with that account from Google Play in the web version of Google Play (it won’t work in the app). Then all in-app purchases will be made with that account. If you previously installed it with a different account, uninstall it and in Google Play (web version, not app) make sure you are signed in with the correct account before installing it again.