Bootstrap theme: editing the registration page
Take a look at drupal api documentation. Customizing forms is quite easy.
https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_form_FORM_ID_alter/7

Just hook into that user_registration_form and for example add into class index bootstrap classes col col-lg-*. Or you can add #prefix and #suffix to wrap things into columns etc. And install Devel module to examine form's structure.

I'm not sure if it possible to use page--user--register.tpl.php to modify layout. Possibly you could create that and add col col-lg-offset-* col-lg-* to whatever element is the container for page[content] variable.