Open Settings, then Customers. The page is titled Self service customers. The route is /app/settings/customers.
What the account is for
A customer signs in to the self-service portal. From there they can view documents, raise requests, and open knowledge base content. A service desk article reaches them only when it is published and marked Show in Customer Portal. A staff role is not required, and a customer is not a user on the internal teams.
You can add customers one at a time or in a batch, edit them, delete them, block a sign-in, or reset a password.
When you need to change it
Managing portal customers covers those actions.