Antonio,

Those are great questions!

The add to user group extension has not meant for a full membership site. Rather it's meant for somebody selling access such as somebody who has lifetime access to a program or bought something online that gives them access to that item. 

If you need an expiration date and the ability to charge them on a yearly or weekly or monthly basis, then what you need is the membership and subscriptions extension which we are almost done finishing to make available for Joomla 4, and 5.

That extension will give you the ability to add them to as many user groups as you wish and remove them from those same user groups. When they are no longer an active paying subscriber, you can set trial subscriptions and much much more. 

As far as licensing goes, we are most likely going to use a similar system that previous j2 store customers have been used to (which provides support for the website the license is linked to.)

Adam