@felix
XenForo is a PHP-based forum software, and to add paid upgrades or user groups, you will need to modify the code and add payment processing capabilities. Here's a general outline of what you need to do:
It's important to note that modifying the code of a software application, especially one as complex as XenForo, can be a challenging and time-consuming task, and it's recommended to have some experience with PHP and web development. You may also want to consider hiring a developer to assist with this task if you're not comfortable with it.