To retain your ranking on the ladder from last year, you must sign up by April 30th. If you are new to the ladder, the sooner you sign up the better given new players are added to the bottom of the ladder in the order they sign up. Those who were on the ladder last year that do not sign up by April 30th will be added to the bottom of the ladder if they sign up after that date. You can always join anytime regardless.
Go here to sign up. Let's have a great summer of tennis!