At the moment the Matrix provider assumes that all users relate to a course and it always uses the course context.
https://github.com/moodle/moodle/blob/master/communication/provider/matrix/classes/communication_feature.php#L740

We MUST store the contextid in the communication table alongside instanceid, and component. We can then use that contextid to determine the context of the user and to determine the correct power level to use.