Hello
The question is in the title…
I have multiple instances of one object in my scene. All this objects are part of the same group object and they all have an object variable called “presence” equal to 0 or 1.
Is it possible to save all the value with one event? 
Use “for each” event.
For each object “MyGroup” :
do = “something” to the variable “presence” of “Mygroup”
God bless you!
The french translation for that event isn’t very clear…