Am I a USGA member when I join the SCGA?

No. While your SCGA membership includes a Handicap Index, it is not a membership with the USGA. Also, if you join the USGA Members Program, this does not result in receiving a Handicap Index. USGA Membership is a philanthropic donation to the USGA. To establish a Handicap Index, you must become affiliated with a golf club.