If the repeater fields name is crew and the subfields name is crew_person_name you would put in
crew_%_crew_person_name.
Then you’d of course need to setup your posts_where filters to reflect this as well! What this does is basically that it looks through all values in post_meta where the meta_key is formatted like crew_number-of-row_crew_person_name (this is how ACF saves repeater field values).
For a post object field the only thing that is actually saved to the DB is the post ID (the post object is fetches through the get_field/the_field functions) so you just need to query for the ID of the volunteer post.
