They didn't hire her directly. They hired her husband, Toussaint Charbonneau, who had reportedly bought her and his other wife, Otter Woman, when they were in their early teens from a warring tribe, the Hidatsa. The two women were from the Shoshoni tribe and had grown up in current-day Idaho before their capture by the Hidatsa so they knew both the language and the geography of the area.

Once they joined the party, Sacagawea distinguished herself with a few brave actions and some navigational help but her biggest material contribution was likely that when they did encounter the Shoshoni tribe, their leader Chief Cameahwait turned out to be Sacagawea's brother and this eased relations between the explorers and the locals and allowed them to barter for horses and a guide to help them across the Rocky Mountains.

Sources: Lewis and Clark Journals Online: http://lewisandclarkjournals.unl.edu/

Sacagawea: From Captive to Expedition Interpreter to Great American Legend- Her Life http://www.bonniebutterfield.com/Expedition.html