Davening Maariv with a Minyan during Pelag Hamincha versus Davening in private after nightfall?
One who normally Davens Maariv after nightfall who is present by a Minyan that Davens Maariv after Pelag Hamincha, is to Daven with them [if a Minyan will not be available later on], and then repeat Shema, without its blessings, upon arrival of Tzeis Hakochavim. However, he is not to repeat Shemoneh Esrei of Maariv even though the Minyan Davened much time prior to night, unless he is very meticulous in all other matters of piety. [However, this law only applies to those who on occasion daven Maariv after Pelag Hamincha, in which case here too they should Daven with the Minyan. However, one who is accustomed to always Daven Maariv after nightfall is not required to Daven beforehand with the Minyan even if this will cause him to not be able to Daven with a Minyan later on. Thus, in the above example, he may delay Davening Maariv until nightfall even though he will have to Daven in private.]
 Michaber 235:1, based on Beis Yosef 235; Ketzos Hashulchan 27:4
 See M”B 235:15
 Michaber ibid
 Rama ibid Mordechai Brachos; Hagahos Maimanis 3; Terumas Hadeshen 1
 Tzemach Tzedek Chidushim Brachos 1:2; Ketzos Hashulchan 27 footnote 12; Gr”a in Maaseh Rav 65, brought in Biur Halacha 235 “Veim”
Other opinions: Some Poskim rule this law applies even to a person who is always particular to Daven Maariv after Tzeis Hakochavim, nevertheless, in such a case he is to forfeit his custom for the sake of Davening with a Minyan. [Shaar Hatziyon 235:16, unlike Maaseh Rav; Igros Moshe 2:60; Piskeiy Teshuvos 235:3; See Toldos Chofetz Chaim who testifies that so was his custom]