Question:
What is the halacha on shabbos when its 1 minute to shkia, 10th man just arrives, do we start laining, or say shmonei esrei?
Answer:
Since one is starting before the shekia and after the sheki – “bein hashmoshos” is also a question of day or night, therefore you should start laining, or praying shmonei esrei.
Sources:
O:CH 233 Mishnah Berurah 14 7 Shar Hatzion 19 &21, Eshel Avrohom (Butshatsh) O:CH 232-233.