Today a customer reported that VSTS is not sending approval pending email if the approver is a VSTS group. Customer also mentioned that the approval email is sent if the approver is an individual user.
I was able to reproduce the problem on my test account and here are the steps that I did to come out of it.
1. Saw the account level notification admin settings and observed that the default delivery option for all groups is set to "Do not deliver".
2. Saw the notification admin settings for my test group and observed that the default delivery option is set to "Do not deliver".
3. Changed the default setting for my test group to "Deliver to individual member" and it worked.