We are currently having issues placing internal contacts into contact groups within our free version of Teams. We get the error: "We couldn't add a buddy to the group." I have encountered this error on both the http://teams.microsoft.com website and on multiple locally installed applications. I have verified that each are also using the latest version of the software. Is there a way to correct this issue?
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: