nextcloud/admin/ajax/togglegroups.php

<?php

// Init owncloud
require_once('../../lib/base.php');

// We send json data
header( "Content-Type: application/jsonrequest" );

// Check if we are a user
if( !OC_USER::isLoggedIn() || !OC_GROUP::inGroup( $_SESSION['user_id'], 'admin' )){
	echo json_encode( array( "status" => "error", "data" => array( "message" => "Authentication error" )));
	exit();
}

$success = true;

$username = $_POST["username"];
$group = $_POST["group"];

// Toggle group
if( OC_GROUP::inGroup( $username, $group )){
	OC_GROUP::removeFromGroup( $username, $group );
}
else{
	OC_GROUP::addToGroup( $username, $group );
}

// Return Success story
if( $success ){
	echo json_encode( array( "status" => "success", "data" => array( "username" => $username )));
}
else{
	echo json_encode( array( "status" => "error", "data" => array( "message" => "Unable to delete user" )));
}

?>