Snape and the Order

[Inge]

No: HPFGUIDX 93735

Yesterday a friend and I got to talking about Snape and when he 
actually joined the Order. Suddenly both of us had doubts about that. 
Help anyone?
Was Snape a member of the Order during the First War leading up to 
Voldemorts downfall? 
Or did every member of the Order at that time at least know about 
Snape switching sides and acting as a spy for the Order?
Or didn't Snape become a member till after Voldemorts disappearing?
Im confused - and I don't want to be....

Inge