How to enable notifications for a Media gallery

Hi All

We have a number of private groups in our community that have a media gallery that holds files for the group members to download. 

However, it's come to our attention that when a new file is uploaded to the gallery, no notifications are going out to inform the group members that a new file has been uploaded. 

How do group members 'sign up' to be alerted when new files are added to the gallery? 

Lisa