Office 365 Distribution groups with Active Directory

How do I create a distribution list that has external email addresses in Active Directory and have it properly sync to Office 365 Exchange online?  We have a notification group that has internal email addresses and external "phonenumber@vtext.com" addresses in it so that when an event happens, we can get a text message and an email.

With just the internal AD users in the group, the email works fine, send an email to the group address and everyone gets it.  

I created an OU for Contacts in AD and then created seperate contacts for the people (who also have AD accounts) and just put in their vtext email address in the email field.  I then added those contacts to the distribution group, but now the group doesn't appear to sync the membership.  How do I go about acomplishing this?  I would like to be able to do it all through my on premise AD and sync it to the cloud.