It is quite simple really - for most events you don't need any form of grading or licence, you can just inform the chief marshal of the event that you would like to attend, they will allocate you a stage and or location on that stage. If you inform them that you have never marshalled before, they will endeavour were possible to place you with experienced marshals who will help and assist you.

The Rally GB, as a very high profile event,  Introduced  legislation that marshals MUST be registered on the NATIONAL MARSHAL REGISTER in order to marshal on this event.