To add a person (or yourself) to a Client Folder,

Step 1 - Go to the Client Folder by typing the name of the client in the top search bar. Or alternatively, selecting the client from the Dashboard

Step 2 - On the client page, click on the Edit button (towards the top right of the page).

Step 3 - In the dialog box that appears, amend the Owners field to add/remove people who own this Client relationship.