Signup code into every post

Could use this http://ww.wp.xz.cn/extend/plugins/simplr-registration-form/

If you want WordPress to place the registration automatically in each post you would add code to the appropriate template (e.g. your theme’s index.php maybe):

<?php echo do_shortcode('[Register]'); ?>

Also see:
Stepping Into Templates
Template Hierarchy

Well if post.php is the template that displays your posts then that would work.