Ordinarily, all are to make a profound bow (from the waist) at these words to show honor and respect to the humility of God who humbled himself to share in our humanity that we might share in his divinity.
All genuflect at the words, and became man.
Only rarely does one see the faithful keep this rubric, even though it is printed on the very page they read, often in the midst of the very Creed they read.
It is even rarer (naturally) that one sees this alteration of the rubric kept, even with a priest or deacon announces it just before the Creed is said (this has happened with me on more than one occasion).
Please, take the lead in your pew and genuflect when you say the words, and spend a moment in quiet prayer, reflecting on the great mystery of the Lord's birth.
Let us then bend our knee on this most holy and solemn of feasts before our only true King and place ourselves under his care and in his love.