When Setting up a meeting, type in the external email address you would like to attend the meeting via teams.

Here is an example of a Gmail account being invited:

Then the specified user email will be sent an invitation to join the meeting you are setting up.

Once the meeting starts, the creator will need to admit the specified external users as a guest when they join.

                            Example: 

Adding External Users to a Meeting already in progress

Once a meeting is already in progress you will need to share a link to join within the participants tab.

Clicking "Share invite" will open a window where you can copy the meeting link and then you will need to send it to any external users you want to join the meeting.

Then all you need to do is admit them like outlined previously or by clicking the green check mark within the participants list where you see the user waiting in the lobby.

Adjusting Permissions for External Users Joining Meeting

You can adjust the permissions to allow external users or any users within the organization to bypass needing to be approved or admitted into the meeting. Once you have the participants   tab open then click on the three dots circled in the image below. From the drop down list           select "Manage permissions".

Then a browser window will open that will allow adjustment of settings on who can bypass the lobby and not require admittance. Defaults to "people in my organization and guests". 

Guests are not the same as inviting an external user. In order to allow external users you invited to join your meeting without admittance from the organizer, the organizer will need to                  set who can bypass the lobby to "everyone".