Post with certain term and without any term

by

This should be possible with the tax_query parameters for WP_Query.

The query is either for posts where any term NOT EXISTS, OR, for any posts where the term IDs are IN the provided array.

'tax_query' => array(
    'relation' => 'OR',
    array(
        'taxonomy' => 'group',
        'operator' => 'NOT EXISTS',
    ),
    array(
        'taxonomy' => 'group',
        'field'    => 'term_id',
        'terms'    => array( 23, 42 ),
        'operator' => 'IN',
    ),
)