Click on the Cases (Projects) tab on the top of the page. In the left panel, select a desired case. On the right side panel, click on "Staff Access/Rates" tab. On the screen, check the checkbox in front of a staff name. An "Edit" link will appear. Click on that link to enter a new rate. Click Update. This process will do two things, first, if the selected staff is not an administrator, the selected staff will now have access to this case (administrators have access to all cases automatically). Second, a new hourly rate (only for this case) for the selected staff will be set.
You may also create case categories and set fixed hourly rates for all cases of a selected case category. These case category wise rates can be set under the Staff tab -> Hourly Rates for Case Categories. Rates for individual staff members are copied to the case to which the selected category is assigned to. If a staff rate is set to 0 in case category wise hourly rates, global hourly rates (or client level hourly rates) for that staff member are copied to the case.