Select the 'Participants' option under your course functions on the left of the page, next you have the option to add the user (see instructions on 'Adding a User') or if the user is already a participant but you are desiring to assign them another role, then follow these next steps.
The page should look something like this.
The 'Participants' page shows the roles of the user, select the pencil to alter the the user's role.
You have a few options to assign the roles of Manager, Non-editing teacher, Teacher's Assistant, Non-Grading Teacher's Assistant and Teacher.
Once you've assigned the appropriate role save changes by selecting the little discette.