Answer
The ability to create folders in the Navigator is bundled into the standard FND_ENDUSER permission set and generally doesn't need a separate explicit grant. For granting access to a SPECIFIC folder that's already been created: if the folder was created while editing a Base Profile directly, it's automatically available to everyone assigned that profile. If instead it was created within an individual's Personal Profile, locate that folder node under Solution Manager > User Interface > User Profiles > (the specific profile), then RMB on that node and select 'Copy Values to Profiles...,' choosing which other users or profiles to copy that folder access to.