When you create a folder you have the ability to auto-assign the folder when you create a new client.
Inside the folder, under option select auto-assign. Then you will click update for it to start assigning this folder to any new client that you add.
*If you create an auto-assign folder, it will only assign to new clients. If you want clients that are already in Coach Catalyst added to the folder, you will have to manually add the client.