Can workers see which Worker Groups I've added them to?

No, workers will never be able to see if you've favorited them, blocked them, or added them to a Custom Worker Group.

Worker Groups are only available to you (the business requester). Workers will never know what groups you have put them in. If you favorite them or add them to a Group, your Shifts will always be visible to them. If you block a worker, any future Shifts you post will not be visible to them.

To learn more about Worker Groups, please see this blog post.