You can look up an individual student and add them to a cohort but the following method is more efficient if you have a bunch to do at once.
Manage Remote and In-Person Cohorts From the System Page
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Right Click: So to right-click all you need to do is use two fingers to tap on the trackpad at the same time. This acts like a right click and
Share Google Docs with Just your own StudentsShare Google Docs with Just your own Students
To do this you need access to the student rosters, here.