Sharing Testcases in Zephyr Scale

Yes, this it possible, depending on what permissions you have applied.

If you do allow this to happen (I do), it's worth knowing that if a test case is changed after it is added to a test cycle, when a tester opens that test case in any test cycle, they will receive a banner message advising that the test case has changed, asking if they would like to update the test case to the latest version.  They can either ignore and execute the steps are currently visible in the test player, or update the test script and execute the new steps.  I've illustrated this way of working in the image below, where the green boxes are 4 separate uses of the same test case in 4 different test cycles.  Updating the test case in the Test Case area could result in all 4 instances of the test case being updated.  Hope that helps!