webinars plus new meeting registration options

No need to create a security group, you simply need to assign the corresponding policy to the users who you want to be able to create meetings with external attendee registration. To do so, you can use the Grant-CsTeamsMeetingPolicy cmdlet. You can also do it via the TAC, but "registration" related settings are not yet exposed there, at least not in my tenant, so best to confirm the values via PowerShell first. Only the default "Global" policy allows Externals, so you might as well create a custom policy if needed.