To add a co-teacher to a course, the first thing to do is go to the course, then select People
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Once here click the people + addon under the Teachers section
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Here you have to either type the co-teacher's email or name at the top, then once all the teachers you want to add are selected press Invite

The co-teacher needs to now go to the email that you selected in the previous list and look for the course invitation email. If they type Invitation to co-teach in the search bar at the top it should show up.
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Once you find the email click the Accept invite button
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It will take you to a new window where you have to select Accept it will also identify what course and email you are accepting for

You can now as the primary teacher see the teachers who accepted your invitation to co-teach the course under People and Teachers.
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