Force Registration in Checkout depending on product

Hi,

I don't see how you will be able to do what you want.

Being able to restrict content based on user groups requires user to register on the website in order to create a user account.
So the "Allow User Registration" setting of the Joomla user manager has to be activated for that to happen.

Now, supposing that you activate that, the rest is easy. You don't want to change the "User group on registration" setting. You can leave it to "inherit".
What you need to do is to configure the "user group after registration" setting of each webinar product to the corresponding user group. That way it will add that user group to the user account of the customer after the purchase of the product.
Note that if you don't see the "user group after purchase" setting in each product, you need to activate the HikaShop group plugin via the Joomla plugins manager.
Also, if you have a recent version of HikaShop, this plugin will force the user to register during the checkout if he has a product with the user group after purchase setting set in his cart. You just need to configure the "Display method for registration" setting of the HikaShop configuration to "selector" and then in the "registration" setting below, you can select both "registration" and "guest".