Sometime, you will want that when users register for an event, they will be assigned to the Joomla group(s) you want so that you can use Joomla core ACL to give them permissions to perform some actions or access to restricted resources on your site (of course, these resources are not available for public and normal registered users). Please follow the steps below to setup it if you want to use this feature:
If you want to restrict access to resouces to your registrants based on Joomla core ACL (you need to understand how Joomla ACL works), this will be a very powerful and flexible plugin.