pewc_filter_product_extra_groups.php

<?php
/**
 * Reorder groups
 */
function prefix_filter_product_extra_groups( $groups, $post_id ) {

	if( empty( $groups ) ) return $groups;

	/**
   * Here's a list of groups that we want to appear first
   * Enter your list of group IDs in the list below, separated by commas
   * Any groups you don't add here will just get added after these groups
   */
	$priority_groups = array( 2803, 5138 );
  
	// This is the newly ordered list of all groups
	$new_order = array();

	if( $priority_groups ) {

		foreach( $priority_groups as $pg_id ) {

			if( isset( $groups[$pg_id] ) ) {
				// Add priority groups to our array
				$new_order[$pg_id] = $groups[$pg_id];
			}

		}

	}

	// Now add the remaining groups in the original list, excluding priority groups
	foreach( $groups as $g_id=>$group ) {

		if( ! in_array( $g_id, $priority_groups ) ) {
			// If this group isn't already in our priority group, then we can add it now
			$new_order[$g_id] = $groups[$g_id];
		}

	}

	return $new_order;

}
add_filter( 'pewc_filter_product_extra_groups', 'prefix_filter_product_extra_groups', 100, 2 );