In the evening, we make two blessings before the recitation of Shema, and two after it.
One should not talk between the blessing about redemption (ge'ulah) in the evening and the Amidah; even those who routinely say 18 verses and "yir'u eineinu" should not pause between that paragraph and the Amidah. What about when the reader announces Rosh Chodesh [so people can remember to say ya'aleh ve'yavo] between Kaddish and the evening Amidah? This is not a pause, since it is a requirement for the Amidah; and thus, one could also say that Barekhu, to exempt one who did not hear it, wouldn't be considered a pause either. REMA: "See above in section 69. I have seen sticklers who stand while saying the 18 verses of "barukh adonai le'olam" etc. (Ps. 89:53). This is a nice custom because it was set in place of the 18 blessings [of the Amidah], and thus one should stand for them like they do for the Amidah."
If when he arrived the community had already recited Shema and was standing to pray the Amidah, he should pray the Amidah with them and subsequently recite Shema with its blessings.
After "shomér amo yisra'él la'ad" [at the end of Hashkivénu], one should say "amen" after that blessing, but should not answer "amen" after the [following] blessing "hamelekh bikhvodo" REMA: "see above at Siman 215."