When granting access to the customer portal, you can send a welcome email to the customer containing the username, password, and a link to the customer portal. You can also see the date and time when the email was last sent.
If no permissions are selected, the customer will have read-only access.
This is what a standard welcome email looks like:
The welcome email can be edited under Settings > Standard Registers > Email Templates > Customer Portal Welcome Email:
The email address under company settings is the sender address for the welcome email that is sent out.