How do I add a specific person to the stage?

You need to be at the stage level to see any roles assigned. The roles are shown as a dark blue circle.
Click on the role to open the sidebar.
In the sidebar, under the Assign staff section you can click on a staff member attached to this role to add them. The list shows staff members who share the same primary office and have the same primary rate that is being selected. If a resource has a red circle, this means the resource is overallocated on other projects. Synergy bases this on all the other bookings the staff member is working on during this time period.
When you click on 'More', you will see staff members who have the same primary office, but secondary rate; and are part of another office but with the same primary rate; and lastly, staff members from another office, using the secondary rate.
Click out of the sidebar to see the changes.