To activate the E-Commerce for all the users with a valid email address in their user profile, just click on Settings and then on: 

A new window will pop up with the following message: 

Click on YES. All users will receive an email with the access link and credentials. The password will be temporary and they can change it on their E-Commerce profile.

In case you cannot see the pop-up window, click on the column with three dots on the right-hand side of your screen (if you are using Google Chrome) and then click on the minus symbol to reduce the zoom.