The first thing you'll need to do is go click on People then on Users, then hit the Bulk User Management button.
Next, click on the Bulk Update (Prefilled) tab and click CSV Bulk Update Template.
Find the CSV file that you just downloaded, and use a file like Excel to open it. Once open, you will need to enter TRUE in the email_notifications column. Once done, save the file.
Next, hit Choose File and select the file you just updated, then click Update Users.
And you're done!