There is NO joining the CMP per se, The first time you deal with them, either by buying something or shooting in a CMP match you are assigned a a CMP Number.
Your competition number and customer number are the same. Don't cost nothing except what you spend in sales or entry fees to the match.
Go to and to read about the CMP, not just sales, not just competitions, but EVERYTHING.
Start with the History when Teddy Roosevelt started the program. Start when congress established the National Match Program.
Study the SMALL ARMS FIRING SCHOOL program. (Personally I think attendance in the SAFS should be a requirement for every citizen of the US, I think it should be part of basic training to all our servicemen/women during basic training)
Look at the SAFS program and you'll see what I mean.