This is the correct link to become a ACMSiggraph member the other one is just for ACM.
It is a year membership for $42.00.
https://campus.acm.org/public/qj/login_gensigqj.cfm?rdr=promo=QJSIG&offering=415&form_type=SIG&CFID=55897949&CFTOKEN=28995519
