Guest access in Microsoft Teams is one of the best features for B2B collaboration available. It allows us to securely bring external users into Microsoft Teams for IM, Calling, meetings and file collaboration.
This is distinct from Federation, where we message and share files with external users directly.

As announced in Microsoft Major Change Notification MC228482, there will be a change in the near future to the Teams Guest Access Default Configuration. Currently, the service default for Guest Access in Microsoft Teams is set to “Off”.

As of February 8th 2021, this default setting will change to “On”. It’s important that if you do not allow Guest access in your environment, to ensure this setting is configured and set to “Off” before the default change takes place.

If Guest Access is already enabled in your organization (or you would like it to be), then no change is required. It is important however that when enabling Guests in Microsoft Teams, that the appropriate controls are put in place, such as MFA, Domain Whitelisting, limitation of the Guest User Inviter role, Azure AD Access Reviews etc.