Until this point, the plot is clear. Now, two important details that affect our understanding of the rest of the story are both missing:
a) Where are the brothers eating?
Considering that they threw Yosef into a pit 'out in the midbar,' most probably they returned back to their camp site in the Dotan area to eat (see 37:16-17). [Besides, it would not be very appetizing to eat lunch while listening to your little brother screaming for his life from a nearby pit; see 42:21 for proof that he was indeed screaming.]
[Later in the shiur we will bring textual proof for this assumption as well.]
b) Where is Reuven?
Considering that Reuven's entire plan is to later save Yosef from the pit, it would only be logical that he either stay near the pit or remain with his brothers (wherever they may be). Certainly it would not make sense for him to go anywhere else, surely not far away!
However, from the continuation of the story we know that he does not stay near the pit, because he returns to the pit only after Yosef is sold! Therefore, if the brothers are indeed eating away from the pit, he must be eating lunch with them! After all, not joining them for lunch could 'blow his cover.' Furthermore, the Torah never tells us that he left his brothers.
Hence, it is only logical to conclude that Reuven remains with his brothers, and they all sat down to eat away from the pit.