@fulvia_musti
When you create a collaboration from within Canvas, you should designate who is a collaborator and Canvas automatically shares it with those people. It is similar to going into to Google Docs, clicking the Share button, and putting in email addresses -- Canvas does that for you. If your email addresses isn't in the list, you don't get access to it.
You do not get a shareable link that allows anyone with the link to edit it (only those who were added as collaborators).
I don't know which type of collaboration you're using, but for Google Docs, Sheets, or Slides, what I do is click the share button inside the Google Application and ask for a link so that anyone with the link can edit the document. Then I post that link into my course.
Okay, that's not totally true, I was tailoring it to your situation. What I do is create a read only or view only link and paste it. Then I require students to make their own copy to edit so that they don't change my original.
Collaborations can be confused with cloud assignments. A cloud assignment is similar to the process I just described and makes it easier to work with. The one time I tried a cloud assignment, my students couldn't access it, so I went back to my known method (that I described) and never tried them again.
More information on collaborations and cloud documents is available in the Canvas Instructor Guide.
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