In your course, go to the People page. You will see a blue button in the upper right that says +People. That is the option you want.
This will bring up a pop-up menu. The easiest way can often be via email (which is what pops up by default). Add their email in the text box. Down below are some settings. You will likely want to use either Teacher or Co-Teacher. This will allow the person to run the course like a teacher. Make sure to uncheck the setting about only interacting with users in their section.
There is more information here: https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-add-users-to-a-course/ta-p/1119
If you are still having an issue, please upload a screenshot showing what you feel is not working correctly so we can better help you. If you do not see Teacher or Co-Teacher options, you will need to talk to your tech support as the users on here will not have access to your Canvas instance to assist.
BTW, this will only add them to your Canvas course. This will not give them the ability to access your Clever links. For that, you will need to talk to Clever, or ask one of your techs to come assist you if they know how to do it.