Some info about Luftwaffe structure:

Luftflotte - aviation fleet
Fliegerkorps - aviation corps
Fliegerdivision - aviation division
Geschwader - air group
Gruppe - air wing
Staffel - squadron
Schwart or Kette - deputy

KampfGeschwader, or KG - bombers group
StukaGeschwader, or StG - dive bombers group
JagdGeschwader, or JG - single-motor fighters group
ZerstorerGeschwader, or ZG - heavy fighters group (Me-110 usually)
LehrGeschwader, or LG - leading pilots group, or elite pilots group

So, if you see designation I/ZG 2 it means 1st Gruppe in ZerstorerGeschwader 2.
If it goes 1/ZG 2 it means 1st Staffel (not Gruppe) in ZerstorerGeschwader 2.