Right-click the containing folder and choose 'Properties'
Click the 'Security' tab.
In top pane, highlight the account you want to give access to that folder.
In bottom pane, tick the appropriate permission (allow or deny) for each action (read, write etc).
Click 'Apply' when finished editing permissions for that user.
Repeat for each user account whose permissions you wish to change (allow or deny).
Click 'OK' when finished using the permissions editor.