If you want to obtain league points you must register. You do not need to register if you just want to race without getting league points.
To register you must complete the form here on the British Cycling website. You must either ride for an affiliated club, or live in Nottinghamshire or Derbyshire to register for the league. League points are not backdated if you take on membership mid-season.
To affiliate your club you need to complete NDCXL Club Affiliation Form 2019.