Where did she come from?

It was Vicki London who actually can be seen in the beginning, when they were dancing in the rain & mud and when they were about to eat the goo (and of course, shows up for one scene at the end as a giant).

Here is what the IMDB trivia states:

Vicki London was originally cast with a larger role, but declined to disrobe in preproduction, so her part was changed with Joy Harmon's, and much of London's subsequent screen time was cut. Her agent kept her in the movie at full pay, while London didn't seem to object to what happened, since she saw the whole thing as forgettable.

She's also briefly visible during the scene where the giant teens approach the park to do their giant dance. I was kinda surprised MST3K didn't make a reference to her absence.