Office 365 Informatiom Barriers allow tenants to erect communication firewalls between different groups. Teams supports Information Barriers, but currently has a problem adding new guest accounts to team memberships. An easy workaround exists, but debugging what’s going on is difficult because of the lack of clues.

Oddly but maybe on a sperate note, I've seen this where you try add a new guest to a security group as part of an existing info barrier segment using the "memberof" attribute and the group is greyed out in Azure AD and EOL but allows me to add the guest to the group in the M365 users portal @Tony Redmond