The Arvit prayer consists of the Shema and its blessings, followed by the Amida. There are situations in which one arrives to the synagogue for Arvit and finds that the congregation is already at the Amida. In such a case the Shulhan Aruch (Orah Haim 236:3) states that one may join the congregation for the Amida, and then go back and recite the Shema with its blessings. In Shaharit there is great value to reciting the Amida immediately after the blessing of “Ga’al Israel” and is known in the Gemara (Berachot 4b) as adjoining Geula, redemption with Tefila, prayer. During Arvit, this adjoining is not as critical and as such, the Bet Yosef says that one may reverse the order in order to recite the Amida with the congregation if one arrives late.
Interestingly, the Kaf HaHaim (O.H. 236:22) has a novel take on this and says that on a Kabbalistic level, it is preferable to recite Arvit in the normal order, even if it means losing out on praying with a Minyan. Rabbi Ben Zion Abba Shaul (Or LeZion, vol. II, ch. 15, § 10) agrees with this approach. Nevertheless, Rabbi Haim Palagi (Ruah Haim, O.H. 236:1 & Kaf HaHaim) and Rabbi Eliezer Papo (Hesed La’alafim, O.H. 236:7) write that there is no issue with reciting the Amida with the Minyan and then reciting the Shema, either on a strictly Halachic basis or on a Kabbalistic lavel. As well, Rabbi Ovadia Yosef (Yabia Omer, vol. IX, Orah Haim § 108) disagrees with the Or LeZion and rules like theShulhan Aruch. The Moroccan Poskim do not mention the position of the Kaf HaHaim, suggesting that the custom is to follow the Shulhan Aruch. Rabbi Baruch Toledano (Kitzur Shulhan Aruch), who usually bases himself off the Kaf HaHaim, actually quotes the Shulhan Aruch’s position, and only afterwards quotes the Kaf HaHaim‘s novel interpretation. He concludes that if one has the ability to find another Minyan which is praying Arvit, that would be optimal. Therefore, there is a strong Halachic basis for someone to follow the Shulchan Aruch and pray the Amida with the congregation and then recite the Shema with its blessings.
Summary: If one starts praying Arvit by the time the Minyan has reached the Amida, one should recite the Amida with congregation and then go back and recite the Shema with its blessings.