Folders are available on the Screens, Docs and Data Sources listing pages in the secure website.
The process to edit a Folder's settings is the same for all, so we'll just use Screens as our example.
1. In the Screens page (App Builder -> Screens), hover over the folder name on the left hand side.
2. Click the cog icon that appears when you hover. This will open the "Folder Settings" dialog.
3. In the "Assign to Users" field, chose the users that you wish to have access to the folder.
Doing this will give them access to all Screens and Docs contained in the folder.
4. Click the "Save" button to save your changes.
Once saved, your changes will be immediately in effect - so any users that added/removed from the folder will see this reflect on their app at next synchronisation.