Just to add a little bit to that, because it works slightly differently now we have the Semi-Automatic GP Scoring system in place.
First thing to say is that if you don't have a BFTA Number, you can still shoot a GP, but you won't shoot in grade and you won't be eligible for a trophy or a raffle prize (there are some great raffle prizes at the GP's!)
Once you have a BFTA Number (see your club secretary to check that one has been ordered for you) then you can start to earn an official grade.
If you shoot in your local region events and you have a BFTA Number, Eagle Eyed Simon Evans (the grading officer) will spot your scores as if by magic and start building a rolling 16 percentage grade for you. Once you have 3 scores recorded you'll be allocated a grade.
If you start shooting the GP series and you haven't got 3 scores recorded (regionally or nationally) you'll start the season as "Ungraded A" which will effectively mean you'll be shooting against A grade shooters at a GP. However... once you have 3 scores, you'll be re-graded so that the next GP you shoot will be at the grade you've earned.
So.. in short.. you can shoot a GP even if you don't have a BFTA Number, but I'd advise you to apply for one as soon as possible. Once you have a BFTA Number, try to get out and shoot some of your regional Winter League events. If you can get 3 in before the start of the GP series, you'll start the series with an official grade.
If you can't do that, don't worry too much, you'll be re-graded during the GP series once you have 3 scores.
Sometimes to speed up getting a grade some shooters will also visit a neighbouring region to shoot one of their Winter League events as a guest. Even guest scores go into the grading officer, so in theory it's possible to get a BFTA Number and an official grade in few weeks.
The GP series is great fun and the atmosphere is really friendly and relaxed, there's no snobbery about grades at all and no one is looked down on. The best way to learn FT is to jump in with both feet and ask questions.