The Talmud records a debate between Rabbi Yehuda and the Chachamim whether the time for the Arbit service begins at Pelag Ha'mincha (approximately 72 minutes before sundown) or at sundown. According to Halacha, one should preferably ensure to recite Arbit only after sundown, but there is room to be lenient and allow a congregation to recite Arbit earlier, if they will be unable to assemble a Minyan for Arbit after sundown. Indeed, many synagogues today recite Arbit in the summertime during the period in between Pelag Ha'mincha and sundown. (Shulchan Aruch Orach Hayim, Siman 233)
Does this leniency apply to people praying individually, as well, or only to people praying in a Minyan?
The Kaf Ha'chayim (work of Halacha by Rabbi Chayim Palachi, Izmir, Turkey, 19th century) cites from the work "Machazik Beracha" (Hida) that only a congregation is permitted to rely on the view allowing the recitation of Arbit before sundown. One who prays privately must pray Arbit only after sundown.
Would there be any distinction in this regard between Friday night and other nights of the week? Is it possible that on Friday night even a person praying individually would be allowed to recite Arbit before sundown?
The Magen Avraham (commentary to the Shulchan Aruch by Rabbi Avraham Gombiner, Poland, 1637-1683) rules that a person praying individually may, in fact, recite the Arbit service on Friday night before sundown. He explains that our recitation of the Arbit service corresponds to the service that was performed in the Temple during the nighttime hours, when the Kohanim would place upon the altar the limbs and fats of the sacrifices that had been offered that day. Now as the Rambam rules (Hilchot Temidin 7:1), the limbs and fats of sacrifices offered on Friday had to be placed upon the altar before sundown. Once Shabbat began, only the special Shabbat sacrifices were allowed to be offered and burnt upon the altar, and therefore Friday's sacrifices had to be placed upon the altar before the onset of Shabbat. Hence, the Friday night Arbit service, which corresponds to the offering of the limbs and fats upon the altar in the Temple on Friday night, may likewise be conducted before sundown. Therefore, even though generally a person praying individually may not recite Arbit before sundown, on Friday night he may.
Summary: The Arbit service should preferably recited only after sundown, but a congregation may conduct the service even earlier, after the point of Pelag Ha'mincha, if it finds it necessary to do so. A person praying individually may not recite Arbit before sundown, except on Friday night.