Why did Sacagawea join the Lewis and Clark expedition?

Sacagawea really didn't assertively join the expedition. It was her husband Toussaint Charbonneau who was invited and hired by Lewis and Clark as an interpreter, and Sacagawea would at first only serve as a companion. However, her skills in interpretation proved superior to her husband's, and Lewis and Clark were impressed. She was also very calm and level-headed in many instances, whereas her husband would often panic so Lewis and Clark learned to trust her guidance more than his.

She became invaluable as a guide in the region of her birth, near the Three Forks of the Missouri, and as a interpreter between the expedition and her tribe when the expedition reached that area. After she gave birth during the expedition to Jean-Baptiste Charbonneau on February 11, 1805 she also quieted the fears of other Native Americans, for no war party traveled with a woman and a small baby.