Add the ability for guests to comment without creating an account. Google Docs and Google Sheets both allow anonymous commenting, so I'm sure it can be done!
Forcing guests to create an account confuses them, especially when you're already working with them in a project management platform. It adds a friction point in your workflow that is unnecessary.
Many clients do not want to sign up for multiple apps when working with an agency, and in our experience, it turns them off to using Poda completely. We have had many clients that would rather leave comments on a Google doc and send us this doc rather than create a new account.