Thanks for reaching out about this in the Community! No worries - we're here to help 🙂
Can you confirm you followed the steps on this support page to set the playlist to collaborative? Also keep in mind that it has to be a playlist you created yourself.
We are currently testing a new desktop experience which might explain why you're missing the feature on your Mac, but it should still be available on your mobile. Could you send over a screenshot of how the app looks on your end, so that we can take a closer look into things?
Keep us posted - We'll be keeping an eye out for your reply.
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"Where words fail, music speaks." - Hans Christian Andersen