Client login is configured from the individual contact, not by creating a shared login for the account.
The contact needs a valid email, an associated account, and an available client role. The administrator needs Manage Users and the applicable access to complete setup.
A pending invitation may show Resend invitation and Update invite settings. An existing login shows Login already enabled. Do not create another contact merely because an invitation was delayed.
This setup provides limited access to cases where the contact is the case contact or explicitly assigned. Broader access is managed through user access settings. Creating or inviting a contact does not automatically expose all company cases or internal content.