Register a custom replacement code and group

register-custom-replacement-codes.php

<?php

add_action( 'groundhogg/replacements/init', 'register_my_custom_replacement_codes' );

/**
 * Register any custom replacement codes
 *
 * @param $replacements \Groundhogg\Replacements
 */
function register_my_custom_replacement_codes( $replacements ) {

	$replacements->add_group( 'my_replacements', 'My Replacements' );

	// code, callback, description, group 
	$replacements->add( 'my_custom_replacement', 'my_custom_replacement_callback', 'Returns the contacts username.', 'My Custom Replacement Code', 'my_replacements' );

}

/**
 * The custom replacement
 */
function my_custom_replacement_callback( $contact_id ) {

	$contact = \Groundhogg\get_contactdata( $contact_id );

	$user_id = $contact->get_user_id();

	// Generate any HTML
	$user = get_userdata( $user_id );

	return $user->user_login;
}