Turns out I was making it more complicated than it needed to be. This did the trick:
$user_query = new WP_User_Query( array(
'role' => 'Member',
'meta_key' => 'last_name',
'orderby' => 'meta_value',
'order' => 'ASC',
'meta_query' => array(
array(
'key' => 'publish_publically',
'value' => true
),
array(
'key' => 'sector',
'value' => $_POST['sector-select'],
'compare' => 'LIKE'
)
)
) );