One other group member responds! About time!
The next Saturday, after the lectures have ended we start to call out to our group members by shouting the group number. So much primitive, but no alternatives either. I found another one as well.
Added both on Facebook. Still no sound from the fourth member. I checked the attendance sheet a couple of times. Seems that this person is absent a lot. Other groups have five people, while we have four. And now it seems that we’re down to three.
Still waiting for the last one 😕