Before the invasion of Russia in June 1941, four Einsatzgruppen were formed, designated with the letters A, B, C, and D. Each Einsatzgruppe was subdivided into a group staff and several "Einsatz-" and "Sonderkommandos". Each was attached to an army group, a group of several German armies, except Einsatzgruppe D. Einsatzgruppe D was assigned to the Ilth Army which later became a part of a 4th army group after the Germans reached the Caucasus.
The following is reprinted here from the evidence on the organization of the Einsatzgruppen: a photocopy of a large chart which was exhibited in the courtroom throughout the trial, an Einsatzgruppen report dated 11 July 1941, and an affidavit of defendant Ohlendorf.
Trials of War Criminals Before the Nurenberg Military Tribunals Under Control Council Law No. 10, Volume IV, Washington, D.C.: U.S. Government Printing Office. p. 89