Use the Filter dropdown above the client list and choose Shared with Me. The list narrows to the matters colleagues have put on your list, and hides the ones allocated to you.
This is a view of what you have been asked to watch, not of what you are allowed to open. Every matter at your firm is open to you whether or not it has been shared; use the Firm views if you want the whole book.
The filter works on all three tabs: Clients, Leads, and Archived. Clear it by clicking the × next to the filter button.