Adding a user to the customer portal is easy. First of all, ensure that a customer has been created as a contact (Add a Contact | Housebuilder Pro Help Centre). Next, ensure they are added to a plot (Adding a customer to a plot | Housebuilder Pro Help Centre). If a customer has been created as a contact, and that contact has been added to a plot, you are ready to grant them access to the customer portal.
In the plot details, where the assigned customer information is, there is a blue link titled "Give access to the portal". Clicking this will then display a small pop-up window allowing you to tick what access to the portal you would like to provide to the customer. Tick the relevant access and click "Confirm". This will then send the invite link to the email address on the contact's details.