"One of the soldier officers on this boat was said to be Clark, a war name from his childhood memory: memories of shooting, houses burning, women dragging their children into the woods to hide from the Town-Burner soldiers whose leader was the dreaded Clark. The name was still a curse in the house of Drouillard's uncle, Louis Lorimier, for Clark had destroyed Lorimier's great trading post in Ohio and ruined him."
Lorimier, a trader, moved his operations to present day Cape Girardeau, Missouri, once Spanish territory, sold to the French and ultimately part of the Louisiana Purchase. After being approached to join the Lewis & Clark expedition, George Drouillard wanted to consult his uncle, Louis Lorimier, but wondered how his uncle would feel if he (George) fraternized with the enemy. According to the novel, Sign-Talker, he found out that it would be just fine:
Of course it was William Clark, George Rogers Clark's younger brother who was half of the Lewis & Clark Expedition. Details about other members of the party can be found in the book, The Men of the Lewis and Clark Expedition. Another source can be found here.
While Meriweather Lewis gathered supplies and organized the expedition from nearby St. Louis, William Clark was on the other side of the Mississippi River, a place they called Camp Dubois, where the soldiers and other Corps of Discovery participants trained for their mission.
"The soldiers who would be going on the journey were still up in the shabby little post at Riviere a Dubois: training, bored, scouring the muddy countryside for bootleg whiskey, getting into fights, challenging their sergeants, getting punished, being deleted from the roster for insurrection then restored when they repented."
On 14 May 1804 the expedition started west from Camp Dubois with one keelboat and two smaller pirogues.
George Drouillard (son of a Shawnee mother and French Canadian Father and born in the Detroit area), York (Captain Clark's black slave), Sacagawea and her infant son were the expedition's people of color. Drouillard, an excellent hunter, was hired to provide food for the expedition as well as serve as a guide.
George Drouillard successfully completed the expedition with Lewis & Clark but was murdered on a subsequent expedition (unrelated to Lewis & Clark) in Indian country after he himself had been tried and acquitted of murder.
The genealogist in me found the probate file of George Drouillard online here at this Missouri Secretary of State site.