A contact identifies a person who works with your client account.
A contact can be stored without an account, but the association is needed for it to appear under the intended client and for client login setup. Link the correct account before using the contact in related case workflows.
Create a separate contact for each individual rather than combining several people in one name field. Keep one client account and select the relevant contact on each case.
Add Client Contacts permission controls creation. Creating a contact is a record-management action; it does not automatically invite the person or give them access to client cases.