How are the career growth opportunities on the HR team at Biogen?

Yes, there are many career growth opportunities. That being said, they aren't being handed out, you must go after whatever it is you're looking for. Networking is strongly encouraged since there are many senior team members that have been with Biogen for 5+ years. This allows you to get a better perspective on how to grow internally. Biogen practices what they preach, you must be Pioneering and Agile in the pursuit of whatever it is you're looking to grow into. In my experience, my managers have always encouraged and applauded my internal growth and ambitions for my future with Biogen.