Go to Functions > use the text Search option or 'Advanced Search' to find the Function you want to assign a Staff person to
Click on the Function name
Click on the 'Manage Staff' button
Click on the 'Assign Staff' button
Search for the person you want to assign and click 'Search'
Check the box next to the appropriate role and click 'Assign'
NOTE: If you cannot find the person you want to assign, click on the correct role link at the bottom of the screen to quickly add a new person and assign them to the role at the same time.