As has been discussed in another thread ...
She is Susan Henning, and she was featured with Elvis in the '68 Special. She's the one getting up close and personal while she and Elvis are listening to other people's music during some close-up shots. You'll find these scenes on the '68 DVD set.
She was also the mermaid in Live A Little, Love A Little during the poolside photshoot scene.
Trivia: when she was younger, she was the stand-in and body double for Hayley Mills in the original Parent Trap movie in 1961. She is seen with her back to camera in some scenes whe Hayley faces the front.
Rumours are that Elvis dated (ie slept) with her.