1) When the cantor says “Barechu etc.”, the congregation answers “Baruch Hashem hamevorach etc.”. Even if he personally did not hear the words of the cantor, if he heard the congregation responding, he may answer with them. (Shulchan Aruch and Mishne Berurah 57)
2) If he heard the cantor himself repeating “Baruch Hashem hamevorach etc.”, he should not say it with him. Instead, he should respond with “Amen”. (ibid)
