I don't see Tituba as someone who actively seeks to lead the girls into witchcraft or convert them into being witches. Rather, I see her as someone who has little in way of social identity. She is a girl of color, a slave, foreign, and is essentially seen by the Salem community in the lowest of lights. The girls actually pay attention to her and she feels validated by this. I think that her involvement with what happened in the forest was guided by the fact that she was included more than she was deliberately leading the girls to witchcraft. She was excited at being included, at being part of an inner circle, of sorts. While she was included, she was asked to share her understanding of "the occult" or spells. Tituba saw this as another example of her being included and being validated socially.
Her involvement with the girls does not strike me as a chance to be a leader of witchcraft as much as it does to simply be included in what is happening. It is here in which Tituba has to be seen as a sad figure, someone who simply wants acceptance in a social order that denies it to her. Miller uses Tituba to show the disastrous price that might have to be paid when human beings are so dependent on social acceptance.