Shortest blogpost that will ever be on this blog. Here is a url you can use to get all guests from your Azure AD, including those you have set userType to “Member” for, for different reasons:
https://graph.microsoft.com/beta/users?$filter=creationType eq 'Invitation' or userType eq 'Guest' or externalUserState eq 'Accepted' or externalUserState eq 'PendingAcceptance'