What exactly are you trying to achieve?
Why do newly registered members need to be added to a new user group immediately on registration?
Additional user groups are typically only used for additional permissions, or styling.
I will answer your question regarding why some administrators need a forum with immediate group promotion after registration. I have an aviation forum for pilots and cabin crew.
During the registration process, I ask users to select a user field criteria that I have created: Pilot or Cabin Crew.
At the end of the registration, the user is added to two groups. The original group is "registered," and the second group is the one selected based on the user field criteria.
This setup is critical to me because the nodes are private.
Pilots should only have access to Pilots nodes, and cabin crew should only have access to cabin crew nodes.
Unfortunately, the immediate group promotion system is not included with Xenforo, and this was an issue for me. I cannot afford for my users to register and then wait for the cron job to be executed in order to access their groups.
So, to solve this problem, I installed an add-on that promotes users immediately upon registration.