If you've been invited to join a team and can't login, you'll need to make sure you access the link in your invite email from a computer and login on the web.
In order to successfully accept an invite to a team, this invite needs to be accepted from the web on a computer. But once you do that, you'll be able to login on mobile and use AG from there.
This feature does not currently work on mobile and you'll keep getting redirected to download the app, which you most likely already have.
(If it's still not working, please reach out to us directly via our Customer Success chat, or email@example.com.)