In the Meetup iOS app, we support logging in through the new user registration form. This is so that returning users who have forgotten that they have an account can sign in with minimal friction.
Unfortunately, there's no way for us to prevent that friction for 1Password users.
I'd like to request a method for the 1Password extension to solve this issue. The method would open an editor for an entry with a given username and domain, and either present the existing password (if there's a saved password) or a password generator (if there isn't).
Since we know the user's requested username (they've just typed it) and the URL (meetup.com), we could use this new method to provide sign-in service through our sign-up form.