Aside from being able to identify the latest portal user who has updated a record, there's also a feature that enables the identification of the last login date in the portal. To set this up, go to:

This feature is only available for the Pro plan and when login is on. Logging in doesn't only mean manually filling out their credentials on the login page. It should also cover every time they skip the login page because their detail is auto-remembered (accessing the portal link while already logged-in).