To share a file or folder select it with the checkbox to the left then click on 'Share'
Here you have two options. 'Create A Link' will generate a URL that can be sent to external users via email. To share with internal users click on 'Share'
Click on 'Add Group/User'
Click in the open area and select your user from the list and click on their name to add them
Next Click 'Add'
Put a check next to the rights you would like to share with the user
Manage - Allows the user to download / update / delete the files or folder you are sharing with them as well as any sub-folders
Link - Allows the user to create a URL to share with external users
Share - Allows the user to share the file / folder with other internal users
Once the rights are granted click on 'Save'
Any files or folders shared with a use will be accessible to them under the 'Shares' tab on the left. Any file or folder that is being shared our will be noted under the Files view.
You can add new users to that share or remove existing users by selecting the file or folder and clicking 'Share' again. To remove users click the 'X' to the right of the user and click 'Save'.