We have three Office 365 Business Users, all of whom have the role Global administrator. In addition, each user has a license for Microsoft Teams Commercial Cloud.
In Microsoft Teams > Teams > Manage Team, everyone has the Role Owner.
In Teams > Chat > Contacts > Favorites > Add a contact to this group, I can select the name of a co-owner, but clicking Add results in the message:
Note that the organization is not using Active Directory.
Best Answer
I've recently had a ticket open with Microsoft for this exact issue and we came across a fix. It seems like the issue is to do with the mail attribute not being filled for users, this happens if you do not have an Exchange license assigned.
We're still using an on-prem mail solution, so we aren't using O365 for Exchange. However, assigning a O365 E3/E5 trial license to the user (which should include an Exchange license) fills in the mail attribute for the user. You can use the graph explorer to query a user's attributes:
https://developer.microsoft.com/en-us/graph/graph-explorer
Use a GET request, beta, and query the user with the following: https://graph.microsoft.com/beta/users/emailaddresshere
We've also found that removing the trial license after a few hours keeps the mail attribute filled, which in turn should resolve this.