I noticed that many times people who are late to Maariv, will say Boruch-HU, after Maariv is over. I have learned that this is incorrect, for by saying boruch-Hu, one is inviting the congregation to Bless H-Ashem, and if one does this after Maariv, then everyone will leave and not bless! What is the proper Minhag?
Barchu should be answered at the begining of Ma'ariv. A Takanah made for the case when a person came late to the davening and missed the Barchu, is that one says Barchu at the end as well. In both places the Barchu is praise for Hashem. Rabbi Ro'i Margalit