Change the default setting for the "edit" access level.

bp-docs-filter-edit-setting-default.php

<?php 
add_filter( 'bp_docs_get_access_options', 'wpsupp_edit_default_access_settings', 10, 4 );
function wpsupp_edit_default_access_settings( $options, $settings_field, $doc_id, $group_id ) {
    // Only change the "edit" setting default
    if ( 'edit' === $settings_field ) {
        foreach ( $options as $id => $settings ) {
            // 90 is the id of the "creator" level. Set that to be default. Others, not the default.
            if ( 90 === $id ) {
                $options[$id]['default'] = 1;
            } else {
                $options[$id]['default'] = 0;
            }
        }
    }
    return $options;
}