There have been a couple of posts regarding the webinar feature coming to Microsoft Teams in the Admin center. The Registration feature is the first feature to land, and it will be default on for all. However there are certain things an admin should know about the feature, and how to control it.
Although the feature is on by default for all, the registration page will only be available for internal users of the same tenant. If a user should need to inviter users from outside the organization, a admin have to delegate a policy allowing this action.
The feature can be controlled with the new- or set-csteamsmeetingpolicy as described here. Use the AllowMeetingRegistration or/and WhoCanRegister parameter as documented.