To help streamline user experience, we would like to build a login / signup flow for users in one form, similar to how NYTimes.com and other platforms do it with a "Continue with" flow.
If the email (or Google auth) that they click exists, they simply log them in (if they have the proper authentication), and if it the account doesn't exist, we add a new field to set their password & add Terms & Conditions and modify the state of the Submit button to sign them up.
We are able to do this for Password logins by making a call to our DB to see if the email exists and according to what we get back, we call the login or signup MemberStack functions, but we cannot do this with Google if they click on the "Continue with Google" button as they haven't provided their email yet before clicking.
We could, theoretically, ask for them email before even allowing them to click on Continue with Google, but that would add a unnecessary step for users and might deter them from creating an account.
We operate a registration wall on a large media site in Canada, so milliseconds matter in this flow as we want to maximize free account conversions.