By Birdview PSA definition, customers are the companies, individuals or even other departments in your organization for which you do some work, such as delivering goods, developing a software product, rendering services, etc. As you create a project in Birdview PSA, you may specify the customer associated with it. One of the benefits of it is that it enables you to narrow down the list of projects by using the customers filter. You will also be able to generate an invoice (Billing Report) for the customer.
In some cases, you might want to give the customer an option of tracking project progress, or adding new tasks, issues or requests, One way to do it is by using so-called guest users who can interact with the system via the Guest Portal, e.g. add issues or requests to a project s/he is a member of, etc.
See Creating customers on how to add customers.