If the member area is on the frontend, your authentication doesn’t even need users and you can apply your custom logic anyway. So basically you would POST the data to the database, check if it fits and then store this in the session. Downside is that you cannot take advantage of all Kirby user related method (checking for existing user, log in etc.)
Alternatively, you could also turn your users from your database into virtual users without Panel access.
I’ve written a recipe PR here: Add cookbook recipe virtual users from database by texnixe · Pull Request #1421 · getkirby/getkirby.com · GitHub (and if you don’t want to edit such users but only need read access, you only need the first part, with some modifications with a separate user type without Panel access.
As far as I can tell that is the same as in this cookbook recipe, only turned into a plugin: Restricting access to your site | Kirby CMS