You can find a list of your staff members in the section called Staff on the left hand side.
If you need to add a staff member to a session navigate to the Project by going to Bookings > Projects > All and click on the project.
Then go to the Sessions tab. If your staff member is running multiple sessions, you can use a bulk action. Highlight the applicable sessions and then click on "Staff - Add" bulk action. Here you can specify the date range. By default, the length of the block will be in these fields, but you can select alternative dates if the staff member will not be assigned to every session in the block. Then select which staff member you would like to add to the session and a staffing type.
If everything looks correct, click Go and you will be taken to the confirmation page. Here you can add a notification of the staff's assigning to the session. Click Save and the staff members will be assigned to the sessions and the colour of the session will reflect that it is staffed. You will also be able to see the session on the timetable of the applicable staff member.