Email Notifications Not Working

When giving a user access to a project, they are not being sent a notification to tell them that they have been added to the project. I have set this notification up in the settings area. So all I should have to do is click “add user” and then “update” the project, and they should get an email, correct?

Unfortunately this is not working. Please could you help me with this as it is a very important part of my use-case.

Thanks!