I have a multi site set up on my main site, where I have billing and invoicing setup for my clients, but I also have a standalone installation on a clients' domain. I am looking to have my client use their already set up username and password for their domain to login to my site so that they can view and pay their invoices. I am interested in using the User Sync plugin to achieve this, but I do not need to have all of my user accounts of my main multi site have an account on my clients domain, nor do I need to have their accounts listed in my clients users area. How can I set up the User Sync plugin so that my client can use their account to access my main multi site with the same credentials? Also, if and when they update their password on their site, will they be able to access my main site with the updated credentials? Please let me know.