If you just want to allow all users to access an issue before it’s published, you need to change just one line in pages/issue/IssueHandler.inc.php file, line 409. Change it to this one:
if (isset($issue) && $issue->getJournalId() == $journalId) {
Note that after doing this, everyone will have access to all issues, no matter if they are logged in or not, or if the issue is published, you can’t control that any more. But the publish action will still move the issue from future ones to published, and the template will still mark the non published issue as a preview.
I am not sure if that’s what you want, but it’s an easy change into the code.
Cheers,
Bruno