I recommend custom user roles. These would enable you to then use the user Segments feature and other permission based features. 

1. Create a custom role for the product
2. Pull your list(s) of users
3. Add custom role to community users who use that product
   1. I would automate this via the API using a tool like Zapier or whatever RPA tool your company has. For example, a simple automation might be:
      1. New row in google sheets / excel
      2. Find user via email
      3. Add custom role (if user is found)
4. Setup regular cadence for updating this
   1. look to fully automate somehow
   2. In the interim, repeat your steps 2 and 3 weekly/monthly

I have two customer segments: buyers and sellers, and I need to ensure that they are identified by email whether they are registered as buyers or sellers on the platform.

We use custom roles, like ​@DannyPancratz suggests. It’s added to our SSO, so customers, partners, and employees are granted those roles when they register. You could definitely do a custom role of buyer or seller - and then a role for customer in general - since members can have more than one role.

In a past community, we used Zapier to assign them.

​@Kgastaldo and how do you add it to your SSO?