It is not said in the post-it, but the meeting point before departure is always at the montparnasse station (south side of paris), between the station and the tower.
Check the 'parcours' section on the pari-roller site for the route, which is different every week

In fact, it's at "place Raoul Dautry".
Anyway you'll find it, look for riders !

Max

thanks for the precision, i never remember the name of the square, tought i pass by at least a couple of times a week.
Anway, you can't miss the riders when you go out of the station through the main doors