An employee can only be a part of one review cycle at any one time. Whether it's an active or scheduled review cycle, they can only take part in one.
This will be why some people will not appear in the participant list or search when you are creating a new review cycle.
If you feel they should be available to add, we recommend reviewing existing review cycle you or your Admin colleagues might have created previously. You can 'Edit' an existing review cycle to remove them from it and make them available for this new review.