אין הצבור אומרים עם ש"צ נקדישך אלא שותקין ומכוונין למה שש"צ אומר עד שמגיע לקדושה ואז עונים הצבור קדוש:

The congregation does not say "Nakdishakh..." [the opening of the Kedusha] together with the prayer leader, rather they should be silent and concentrate on what the prayer leader is saying, until [the leader] reaches Kedusha [proper], and then the congregation answers "Kadosh".

It is proper is pay attention to one's feet [i.e. keep them together] at the time when one is saying Kedusha with the prayer leader. Gloss: And people should raise the eyes on high at the time at which they are saying Kedusha, and also move their body and lift themselves from the ground [Tur and Shibolei Leket]. And one should not speak in the middle of Kedusha [Mahari"l and Beit Yosef in the name of Mahari"a and Derech Eretz Zuta]. And one who [already] said the order of the Kedusha and came to the synagogue and found the congregation answering Kedusha should repeat it and answer with them [Responsum of the Rashb"a - siman 249]