In Invoicing Settings Invoice Groups, while adding a new group (+ Add new group), or editing an existing one :

Invoicing data section, Addition field
It is recorded for each invoice. You have to use these variables: $USERNAME$ (Client name) and $PASSWORD$ (Password).
Supplement text may look like this: You may view all your issued invoices also in electronic form on client portal, your registration data: Client name: $USERNAME$ Password: $PASSWORD$