One of the prayers that can only be recited with a minyan is Barechu (“Bless”), a call to communal prayer. Both during the morning Shacharit prayers as well as the evening Maariv prayers, the leader says out loud, “Barechu et Hashem hamevorach” (“Bless the L‑rd who is blessed”), and the congregation responds, “Baruch Hashem hamevorach le’olam va’ed” (“Blessed be the L‑rd who is blessed for all time”).

All congregations recite it before the Shema in the morning and evening, but only some repeat it after the prayers as well. Some always repeat it at the end of both Shacharit and Maariv, while others only repeat it at the end of one or the other. Some only do so on days when the Torah isn’t read. Some omit it on Shabbat and holidays, and others (including Chabad) only repeat it after Maariv on Friday night.

To get some understanding of the varying customs, let’s begin by explaining why some repeat it at all.

Why Some Say Barechu Again

The basic reason for repeating Barechu is for those latecomers who didn’t hear it at the beginning of the services. Accordingly, in some communities, only one who actually missed Barechu would say it out loud at the end of the services. Barechu is also said at the reading of the Torah, which is why some have the custom not to repeat it after prayers in which the Torah was read, since the latecomers presumably heard it then. Others don’t repeat it on Shabbat or holidays since people aren’t usually late at those times (see below regarding Friday night).

In other communities, the custom became to always repeat it, regardless of whether there was someone who actually missed it. According to some, the reason for this is Kabbalistic.

Why Others Don’t Do Barechu Again

Barechu is a call to bless G‑d, the official start of the prayer services. Therefore, some teach that it is not appropriate to say it after the services, unless there is someone in the synagogue who is actually about to begin the blessings before the Shema service (Birchat Keriat Shema). Otherwise, the sight of a congregation being called to pray and then exiting the synagogue has a slight appearance of heresy.

Those who do repeat Barechu counter that although it is true that Barechu is a call to bless G‑d, the congregation's response, “Baruch Hashem hamevorach l’olam va’ed, “ suffices as a reply to that call.

Why Some Only Do It on Friday Night

As mentioned, many, including Chabad, only repeat it on Friday night, irrespective of whether someone came late. This is based on the teaching of the Arizal, who says that the extra soul that every Jew gets on Shabbat enters during the recitation of Barechu, and if one missed the first Barechu, it enters during the second Barechu.

Some add that this is the only evening when the Amidah is repeated (albeit in a truncated form) so that latecomers can catch up and leave the synagogue together with everyone else. On this night, when we accommodate latecomers, it is also fitting that we repeat Barechu for them.

As you can see, there are good reasons and ancient sources for each of these customs, and one should follow the practice of their community.