When creating a new sighting, build a list of witnesses from a select onchange action. Create a new property for the list of witnesses. On create, add the objects to the sightings property witnesses.

While on the page, make the collection of selected witnesses render as a list with a Remove button. Use the select element as a way to add to the list while removing the options from the select element. You will need two actions: addWitness and removeWitness.