Mail enabled security group: Sorry, you are not allowed to share this with external users

Hi everyone

I am trying to configure Alerts on a Discussion Board in SP2013 and running into a "Sorry, you are not allowed to share this with external users" error when trying to add a mail-enabled security group to the Sends Alerts To field.  I would very much like to not list each member individually because then as members change I also need to change this instead of having it happen automatically. 

I did just mail enable the security group so I restarted IIS thinking maybe it hadn't replicated yet or something but no such luck.

Can anyone offer any insight?

Thanks!

Ryan