RE: Dynamics 365 Portal: agent login and search by product
Hi JasR,
Dynamics 365 Portal user can have 2 Type of account--Local Account and External Account.
Local Account:
A user whose control is within CRM is a local account, Here login process has nothing to do with third party application to allow user login in to portal.
Users can be in two Entity of CRM.
Entity 1: user (The Actual CRM license Users)
Entity 2: contact (The Record of contact entity), You don’t have to create and assign license for allowing your users to access portal.
'Register' and 'Redeem invitation'

You can refer following link, which provides more information about it:
How Login Works in D365 Portal – Friyank Parikh Blog (wordpress.com)
For second question, it's hard to filter knowledge articles based on related products, even create a separate product selection dropdown.
Maybe you can add product name to 'keywords' field of the knowledge articles, then you can search it by using product name.

Regards,
Leah Ju
Please mark as verified if the answer is helpful. Welcome to join hot discussions in Dynamics 365 Forums.