This screen displays all users and every folder and permission for each user. Nothing can be edited on this screen. To assign access you should use the Access screen on each individual user or use the Access screen for an individual folder.
Clicking a username will take you to that user's Access screen, where you can change their access settings.
Clicking a folder name will take you to that folders's Access screen, where you can change access settings for anyone in the folder.
This button will export the entire screen contents to a CSV file.