To create a meeting invite where external guests can join, no special permissions are required.

Simply create a new meeting, click the Teams Meeting button (which generates a new Team meeting ID),

and include the guest as a required/optional contact for the meeting.

They will be able to join the meeting, regardless if they use Teams already or not.

If they do NOT use Teams, they will be prompted to download and install Teams, or use the web version. 

It is recommended that they use the web version in those instances (unless they are going to host or share their screen).

Screenshot indicates what was described (this appears when clicking "New Meeting" from the calendar window).