70:3 If a person comes to shul for Maariv and finds the congregation saying Shemoneh Esrei, even if it is not yet night but it is after plag hamincha, he should daven Shemoneh Esrei with them. He should say Shema and its brachos later, after it becomes night. (All this is only if he does not have another minyan he can attend – Mishnah Brurah 236:12.) If the congregation is in the middle of Shema or its brachos, if he has the time to say until “Shomer amo Yisroel la’ad” (i.e., the end of the second bracha following Shema) before the congregation reaches Shemoneh Esrei, he should do so, skipping the prayer “Baruch Hashem l’olam amen v’amen” in order to catch up with them. This prayer need not be made up after Maariv. If he has not yet davened Mincha, he should say Shemoneh Esrei for Mincha while the congregation is saying the Shema and its brachos, take a brief pause, then say Shemoneh Esrei for Maariv immediately after along with the congregation. He should say then say Shema and its brachos later, when it is night.
70:4 “Baruch Hashem l’olam amen v’amen” through “Yiru eineinu” should be recited seated. One may not interrupt from “v”Hu rachum” (which precedes Barchu) until after Shemoneh Esrei. It is not considered an interruption when the gabbai announces things like “Yaalev v’yavo!” or “Tal u’matar!” since these announcements are necessary for the prayer service itself. (Even so, such announcements may only be made between Kaddish and Shemoneh Esrei, and not in Shema or elsewhere – Mishnah Brurah 236:7.)