There seems to be a glitch with the loyalty items too(like if you've joined for 5 years of altador cup it gives you a prize for 3 years), so unless you're playing on a new account no point in joining quite yet
also, sign-ups close June 6th I read? So sign up on all of your accounts by then
I got the two year prize, but I've been doing the AC for 4 years not counting this year. If I count this year it'd be 5 years. I beleive this same issue happened last AC as well.