If you want to give an existing member 'Coach' or 'Admin' access permissions, its best to promote them to Staff rather than trying to set them up as staff from the staff section.
Go to the particular Members detail page
Click 'Promote to Staff' button in right column
Fill out the appropriate parameters on the form. Make sure you select 'Coach' or 'Admin' as the role and decided whether or not to grant them Control Panel access.
Hit 'Save Changes'