The Mechaber, though, writes (Orach Chayim 489:3): “If an individual prayed [Ma’ariv] with the congregation when it is yet day, he counts with them without a blessing, and if he remembers at night he blesses and counts.” The Rema adds that even if this individual answered “Amen” to the blessing, he counts again with a blessing at night if his intention was not to discharge his obligation at the earlier time.
The Taz (ad loc.) takes issue with the Mechaber’s position. He asks: “Is our discussion concerning wicked desecrators [who violate the rules of Sefirat HaOmer by counting while it is yet day]?” He suggests that that the Mechaber is referring to someone who davened bein ha’shemashot, which is safek layla and can possibly therefore be considered evening.
Indeed, at the end of Rabbi Feinstein’s discussion on this matter, citing some of these sources, Rav Feinstein posits that it is more reasonable to agree with those who require one to count after shekiah, even though Ma’ariv may be prayed immediately after Plag HaMinchah. Accordingly, one may still count the Omer of the previous evening even after the congregation has prayed an early Ma’ariv.
In the merit of our fastidious performance of this mitzvah, may it be His will that we soon fulfill the complete mitzvah and bring our Omer to the third Beit Hamikdash, speedily in our days.