Create client
You can create client by clicking on the "Clients" tab on Invoicing. If you activated CB, the fields below "Name" will be saved on Community Builder. You have 3 options. You can create a new Joomla! Account on your website with the client, you can assign an existent Joomla! account or just register the client on Invoicing. You can find tutorial how to do it here ( I'm I want to create a client and he already exists in the list of my user table. ).
Others are similar, the only thing you need to be aware of is the form that appears when you click on the checkbox "create a new Joomla! Account", the only restriction here is to write an email adress that is not in your "User" tables yet otherwise you won't be able to save it. You must add a password too, if you modify some data of the client later, you can let the field blank so his password won't be modified.