How to allow user registration and allow them to upload videos?
If you have to upload a Youtube video in your website, there is the Youtube field module which enables you to upload videos from Youtube by putting the link in the field.

If you want to upload a video from any site use the Media module for this.


  The YouTube field module provides a simple field that allows you to add a youtube video to a content type, user, or any entity.
  
  Display types include:
  
  
  YouTube videos of various sizes.
  YouTube thumbnails with image styles.
  
  
  This module is a lightweight alternative to Media or Embedded Media Field, if you're looking for a way to add video fields from more than one provider, you may want to consider looking into either of those two modules.





  The Media module provides an extensible framework for managing files and multimedia assets, regardless of whether they are hosted on your own site or a 3rd party site - it is commonly referred to as a 'file browser to the internet'.
  
  Media is a drop-in replacement for the Drupal core upload field with a unified User Interface where editors and administrators can upload, manage, and reuse files and multimedia assets. Any files uploaded before Media was enabled will automatically take advantage of the many of the features it comes with.