The Salem Weekly Advocate, 5-23-61, has a letter from the 21st IL of 5-17 datelined "Camp Grant." At the 5-15 organization of the regiment, U. S. Grant mustered in the troops, and the camp was then renamed from Camp Cunningham (after the local qm) to Camp Grant after the mustering officer.
The Mattoon Gazette, 5-17-61 says S.S. Good of Decatur was elected colonel at the mustering. The mustering officer was "Col. Grant." "The name of the encampment has been changed to Camp Grant, but we know not wherefore."
The Mattoon Gazette, May 30, 1861, has a report from "Camp Grant."
Hope this helps!