In the lecture, you have the following three sentences
Mes amis, merci à tous d'être venus.
Sarah, merci d'être venue.
Paul, merci d'être venu.
In all these three sentences, "Mes
amis", "Sarah", and "Paul" are NOT the objects of the
verb "venir", so even they are proceeds of the verb, why should the
verb venir have to in agreement with "Mes amis", "Sarah", and "Paul"?