Mary,
Groupings does not appear to be enabled (have sent video link to administrator), so I usedthe "locally assigned roles" tab to select those who attended as "students" for the assignment. I think the other students can see the link, but do not get the form allowing them to upload their files. This will effectively preclude those not attending from submitting files in the wrong place. It would be better if they couldn't see it on the main course page, but we can't have everything.
Thanks for your suggestion.
Paul