The Appomattox were not a group but rather refer to the Battle of Appomattox Court House, which took place in April 1865 during the American Civil War. This battle culminated in the surrender of Confederate General Robert E. Lee's Army of Northern Virginia to Union General Ulysses S. Grant. This event effectively marked the end of the Civil War, as it led to the collapse of the Confederate resistance. The surrender took place at the Appomattox Court House in Virginia, which has since become a symbol of reconciliation and the end of the conflict.
If you are asking about Lee's surrender to Grant at Appomattox Courthouse, it was April 9, 1865.
Appomattox Court House, which despite the name was actually a small village rather than a single building. The specific building where the surrender was concluded was the home of Wilmer McLean.
Grant re-joined the army when the Civil War broke out, and was made an officer . He started fighting in MO, won some battles and was promoted. He gained control of the Mississippi River and was promoted to major general and given command of the entire union army. In due time, he conquered Richmond and Lee surrendered. He is generally given credit for winning the wara.