The way to enroll a course leader into groups depends on whether they have access to all groups or only select groups. Users should have access to all groups if they are responsible for mediating, grading, and working on material for the entire class. They should have access to select groups if they are only responsible for mediating, grading, or working on material for a particular group.
Add staff that are working with select groups the same way you add users. However, if an administrator enables the Can be auto enrolled into groups permission, users that lead courses and assist users (like teaching assistants) are added to groups at the same time as regular users; this could mean that all or most of your staff end up in the same group. Make sure you even out enrollment using the Enroll Users page.