To add partners and internal users to the customer Experience site, the recommended way is to enable the partner accounts and the internal accounts as customers; the usersunder these accounts will then automatically gain access to the customer site. A customer account is an account that has the Is Customer checkbox enabled.
This indicates that the account is part of your customer network and can access your customer Experience site. To enable an existing account as a customer, you need to edit the account record and check the Is Customer checkbox. This will automatically grant access to the customer site to all the users under that account, without creating new user recordsor profiles.