It is not currently possible to directly share a rubric in Canvas, although this functionality may be phased in during the forthcoming Rubrics Re-Design scheduled to begin in April 2024.
A potential workaround is to make an assignment (the details of the assignment can be left mostly blank) and attach a rubric to that assignment (How do I add a rubric to an assignment?)
Then, that assignment can either be shared with another user via Direct Share or added to Canvas Commons. See the relevant Canvas Guides for details on those functions:
This 3-minute video demonstrates both of these methods: https://framingham.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=14c98d72-f09d-496e-84a0-b132013ff17e