Each client that you add to Introbills will receive an activation link via email that they need to visit and on that link setup their password which they will use to access the client center.
Here's how the activation email looks like. It's possible that the email will end up in the spam folder, depending on which email provider you're using, so make sure to check there before coming to a conclusion that the email was not sent.
This is the first and only step of the client account setup where you need to type in and then confirm the password that you'll be using to access the client center. Another email that you as a client will be receiving when Introbills account administrator sends an invoice your way is the invoice notification email. PDF of the invoice will be sent as an attachment to that email. Keep in mind that you will also be able to login into the customer center on the sub-domain of the Introbills account administrator that added you as a client.
Customer client center gives you overview over all the invoices that the Introbills account administrator who's client you are sent to you. Here you can also keep track of their status.