connections.php

<?php

add_action( 'mb_relationships_init', function () {

    MB_Relationships_API::register( array(
        'id'   => 'programs_to_updates',
        'to' => 'projects',
        'from'   => 'updates',
    ) );

    MB_Relationships_API::register( array(
        'id'   => 'users_to_posts',
        'to' => array(
            'object_type' => 'user',
            'meta_box'    => array(
                'title'       => 'Program access',
                'field_title' => 'Select projects',
            ),
            'field' => [
                'query_args' => [
                    'number' => -1 // This
                ]
            ]
        ),
        'from'   => array(
            'object_type' => 'post',
            'post_type'   => 'projects',
            'meta_box'    => array(
                'title'         => 'Clients with access',
                'context'       => 'side',
                'empty_message' => 'No users',
            ),
        ),
    ) );
} );