Depending on time slot, looks like 100-250 riders.
Can someone explain to me how do you join the pen and then leave to do your warm-up and rejoin in the same spot?
You literally join the pen, then quit the ride. Go do a warm up, then when you re-join the pen you're still sitting there nicely at the front.
Feels like a bug in Zwift to me, I think you should either stay in pen to hold place, or if you quit then lose your place ... but as it allows this then I do this all the time