Stopping Guest Users Getting Into Classified Office 365 Groups and Teams

This Petri.com article tells you how to set a policy on confidential Office 365 Groups to stop owners being able to add guest users to the group (or team).

Removing Email Addresses from Office 365 Groups

Like all mail-enabled objects, Office 365 Groups can have multiple proxy addresses. Microsoft has fixed a bug in the Set-UnifiedGroup cmdlet so that you can remove proxy addresses from groups, but take care before you do.