// Auto-activate users after registration
function my_disable_activation( $user, $user_email, $key, $meta = '' ){
	// Activate the user
	$user_id = wpmu_activate_signup( $key );
	wp_set_auth_cookie( $user_id, true, is_ssl() );
	wp_redirect( /*redirect to */ site_url() );
	exit;
}
add_filter( 'wpmu_signup_user_notification', 'my_disable_activation', 10, 4 );