As a member of SIR, you also need to be a member of RUSA (Randonneurs USA), the national organization. The benefits of the $20 annual RUSA membership include a very informative guide/handbook to randonnuerring and a glossy, quarterly newsletter. With RUSA membership you become eligible to earn a number of awards. Also, if you would like to cycle the Paris-Brest-Paris ride someday, the French insist that you be a member of RUSA before riding your first qualifying brevet.
Another reason to join RUSA is that it is a good thing to do. The organization is vital to promoting the sport in the US. Behind the scenes, they also provide insurance to the local clubs and process all of the results.
Their website where you can download a membership form is www.rusa.org