An error occurred while saving the commentJohn Gwinner commented
The documentation mentions that it's possible, "Start typing a name, distribution list, security group, or Office 365 group to add to your team. You can also add people outside your organization as guests by typing their email addresses."
I just opened a ticket based on the docs, and they said it may or may not work based on some back end attributes. These are not currently available to anyone but MS Operations. So, it sort of works, sort of doesn't, and they are working on it.