module_invoke/module_invoke_all with multiple arguments
use

simplenews_subscribe_user($mail, $tid, $confirm, $source, $preferred_language);


or

module_invoke('simplenews', 'subscribe_user', $mail, $tid, $confirm, $source, $preferred_language);


module invoke is parsing all given arguements and handles them to the called function.

see php documentation regarding passing variables by reference, if that might bug you someday: http://php.net/manual/en/function.call-user-func-array.php#refsect1-function.call-user-func-array-notes