If your colleague isn't already enrolled in your course, you'll need to enroll them.
From your course page, click Participants.
From the Participants page, click Enroll users.
From the Enrollment options box, search for the person that you'd like to enroll in your course by name. The search field is across from Select users. When you see the name that you're looking for, click it.
The name of the person will be added above the search field in blue. You can add more than one person at once. Click enroll users.
Once you are taken back to the Participants screen, Look for the small pencil in the Roles column, alongside your colleague's name.
Let's say you wish to add a teacher role to this colleague. In this case, after clicking on the small pencil, start typing the word teacher. When teacher appears as an option in the small dropdown menu, click it. Teacher will be added in blue just above the text field.
Once Teacher is added in blue above the text field, click the small save icon just beneath the text field. This will save your change to this person's role within the course.
That's it! You've now enrolled a colleague in your course and given them teacher access to your course!