המנהג הפשוט בכל תפוצות ישראל לומר ברכו לפני פריסת שמע. ואין לאומרו בפחות מעשרה שכל דבר שבקדושה אינו בפחות מעשרה (אבודרהם). It has become a universally-accepted custom to recite Barechu before proceeding to the blessings of Shema. Barechu, like Kaddish and Kedushah, cannot be recited unless a quorum—a minyan of ten adult males—is present. Barechu is also said by one who is called upon to read from the Torah. The practice of saying Barechu is mentioned numerous times in the Talmud. It is even cited in the Mishna (Maseches Berachos 49b) in the names of Rabbi Akiva and Rabbi Yishmael.
ואומר שליח צבור: The Chazzan says:
בָּרְ֒כוּ אֶת יְהֹוָה הַמְּ֒בֹרָךְ: Bless Adonoy Who is blessed.
ועונין הקהל: The Congregation responds and the Chazzan repeats:
בָּרוּךְ יְהֹוָה הַמְּ֒בֹרָךְ לְעוֹלָם וָעֶד: Blessed is Adonoy, Who is blessed forever and ever.
אין להפסיק כלל בין ברכו ליוצר אור אפילו לדבר מצוה (לבוש). The two blessings יוֹצֵר אוֹר “He Who forms light” and אַהֲבָה רַבָּה “Great love,” are referred to as the blessings preceding Shema (Maseches Berachos 11a). According to Rambam they were formulated by Ezra and his Court (Laws of Shema 1:7). You may not interrupt your prayers from this point on until after the Shemoneh Esrei. But you may answer “Amein” to the blessings of הָאֵל הַקָּדוֹשׁ “The holy Powerful One” and שׁוֹמֵעַ תְּפִילָּה “He Who heeds prayer;” say the thrice-repeated קָדוֹשׁ “Holy…” and בָּרוּךְ כְּבוֹד “Blessed is the glory…” of Kedushah; and answer Barechu. You may also answer אָמֵן יְהֵא שְׁמֵהּ רַבָּא “Amein, May His great Name,” when you hear Kaddish.” You may also answer Amein after the blessings of יוֹצֵר הַמְּאוֹרוֹת, “Who forms the luminaries,” and הַבּוֹחֵר בְּעַמּוֹ יִשְׂרָאֵל בְּאַהֲבָה, “Who chooses His people Israel with love.” For a more detailed exposition of these laws, see Metsudah Kitzur Shulchan Aruch, Vol. I, pp. 172-176.