This is the complete class for the BuddyPress Codex article : Group Meta Queries: Usage Example. You can test it copying it in the functions.php of your active theme

bpgmq.php

<?php

/* you can copy & paste from here */

//it's important to check the BP_Group_Extension is available
if( class_exists( 'BP_Group_Extension' ) ) :
/**
 * This is a quick and dirty class to illustrate "bpgmq"
 * bpgmq stands for BuddyPress Group Meta Query...
 * The goal is to store a groupmeta in order to let the community administrator
 * feature a group.
 * Featured groups will be filterable from the groups directory thanks to a new option
 * and to a filter applied on bp_ajax_query_string()
 *
 * This class is an example, it would be much better to use the group extension API
 */
class bpgmq_feature_group {

	public function __construct() {
		$this->setup_hooks();
	}

	private function setup_hooks() {
		// in Group Admin UI, you add a new metabox to display a checkbox to featured the displayed group
		add_action( 'bp_groups_admin_meta_boxes', array( $this, 'admin_ui_edit_featured' ) );
		// Once the group is saved you store a groupmeta in db, the one you will search for in your group meta query
		add_action( 'bp_group_admin_edit_after',  array( $this, 'admin_ui_save_featured'), 10, 1 );

		/* The groups loop uses bp_ajax_querystring( 'groups' ) to filter the groups
		depending on the selected option */
		add_filter( 'bp_ajax_querystring', array( $this, 'filter_ajax_querystring' ), 20, 2 );

		/* finally you create your options in the different select boxes */
		// you need to do it for the group directory
		add_action( 'bp_groups_directory_order_options', array( $this, 'featured_option' ) );
		// and for the groups tab of the user's profile
		add_action( 'bp_member_group_order_options', array( $this, 'featured_option' ) );
	}

	/**
	 * registers a new metabox in Group Admin UI, edit group panel
	 * @uses add_meta_box() to register our meta box
	 */
	public function admin_ui_edit_featured() {
		add_meta_box( 
			'bpgmq_feature_group_mb', 
			__( 'Featured Group' ), 
			array( &$this, 'admin_ui_metabox_featured'), 
			get_current_screen()->id, 
			'side', 
			'core' 
		);
	}

	/**
	 * Displays the meta box
	 * @param  BP_Groups_Group $item the group being edited
	 * @uses   groups_get_groupmeta() to get the featured attribute of the group
	 * @uses   checked() to eventually add a checked attribute if the group is featured
	 * @uses   wp_nonce_field() for security reasons
	 */
	public function admin_ui_metabox_featured( $item = false ) {
		if( empty( $item ) )
			return;

		// Using groups_get_groupmeta to check if the group is featured
		$is_featured = groups_get_groupmeta( $item->id, '_bpgmq_featured_group' );
		?>
			<p>
				<input type="checkbox" id="bpgmq-featured-cb" name="bpgmq-featured-cb" value="1" <?php checked( 1, $is_featured );?>> <?php _e( 'Mark this group as featured' );?>
			</p>
		<?php
		wp_nonce_field( 'bpgmq_featured_save_' . $item->id, 'bpgmq_featured_admin' );
	}

	function admin_ui_save_featured( $group_id = 0 ) {
		if ( 'POST' !== strtoupper( $_SERVER['REQUEST_METHOD'] ) || empty( $group_id ) )
			return false;

		check_admin_referer( 'bpgmq_featured_save_' . $group_id, 'bpgmq_featured_admin' );
		
		// You need to check if the group was featured so that you can eventually delete the group meta
		$was_featured = groups_get_groupmeta( $group_id, '_bpgmq_featured_group' );
		$to_feature = !empty( $_POST['bpgmq-featured-cb'] ) ? true : false;

		if( !empty( $to_feature ) && empty( $was_featured ) )
			groups_update_groupmeta( $group_id, '_bpgmq_featured_group', 1 );
		if( empty( $to_feature ) && !empty( $was_featured ) )
			groups_delete_groupmeta( $group_id, '_bpgmq_featured_group' );
	}

	public function filter_ajax_querystring( $querystring = '', $object = '' ) {

		/* bp_ajax_querystring is also used by other components, so you need
		to check the object is groups, else simply return the querystring and stop the process */
		if( $object != 'groups' )
			return $querystring;

		// Let's rebuild the querystring as an array to ease the job
		$defaults = array(
			'type'            => 'active',
			'action'          => 'active',
			'scope'           => 'all',
			'page'            => 1,
			'user_id'         => 0,
			'search_terms'    => '',
			'exclude'         => false,
		);

		$bpgmq_querystring = wp_parse_args( $querystring, $defaults );

		/* if your featured option has not been requested 
		simply return the querystring to stop the process
		*/
		if( $bpgmq_querystring['type'] != 'featured' ) 
			return $querystring;

		/* this is your meta_query */
		$bpgmq_querystring['meta_query'] = array(
			array(
				'key' => '_bpgmq_featured_group',
				'value' => 1,
				'type' => 'numeric',
				'compare' => '='
			)
		);
		
		// using a filter will help other plugins to eventually extend this feature
		return apply_filters( 'bpgmq_filter_ajax_querystring', $bpgmq_querystring, $querystring );					
	}

	public function featured_option() {
		?>
		<option value="featured"><?php _e( 'Featured' ); ?></option>
		<?php
	}

}
		
/**
 * Let's launch !
 *
 * Using bp_is_active() in this case is not needed
 * But i think it's a good practice to use this kind of check
 * just in case :)
 * 
 * @uses bp_is_active([component]) to check the group component is active
 */
function bpgmq_feature_group() {
	if( bp_is_active( 'groups') )
		return new BPGMQ_Feature_Group();
}

add_action( 'bp_init', 'bpgmq_feature_group' );


endif;

Is there a way to turn this into a plugin? I tied adding the standard header, but when activated, the feature didn't work like it does in the functions.php.