Log into your Moodle course.
Turn "Editing" on if it is not already on. You turn it on by clicking "Turn editing on" in the upper right corner.
Click on the closed eye "Show" icon under "People in the upper left corner of the page to display "Participants."
Click on "Participants."
Click on the blue page and pencil icon to the left of the title, "All participants."
Click on the "Role" you will assign the new participant.
Add the participant and then click on their name in the "existing users" column.
Click on the box to the right of the column so that a check mark appears. If you leave your mouse over the box, it will say, "Hidden Assignment."
Click "Assign roles in Course: XXXXXX" where the X's are the name of your course.