How do I add another instructor to a course?
This Canvas doc How do I add users to a course? will likely be helpful here. I know in our implementation of Canvas we lock courses a year after their conclusion and after that point the change would need to be done by an account admin.
If you can see the +People button isn't greyed out, and you can click on it, you are able to add individuals to the course; unless that ability has been restricted in configuration.
If you have any questions I am happy to help, but your local Canvas admins may be a better resource for this as I don't know how your instance of Canvas has been configured.
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