Any client you add to TIDY that you already work with is an existing client and considered a private client, and any client that books directly with you on your TIDY website is also your private client. The only clients that are not private clients are Clients from TIDY.
It's free! There are lots of features to help with scheduling, re-scheduling, to-dos, preferences and more. Another big benefit is that adding all your existing & private clients to one schedule gives you a single schedule of availability. Your clients can see this update in real time so they know when they can schedule with you. You also have the option of getting Clients from TIDY to fill in unused times in your schedule.
No, this is just a tool for you to use. Your private clients don't need to do anything.
Yes, managing private clients is free. Credit card processing is a paid feature if you use that, but you do not have to.