Why did Avimelech come to visit Yitzchak?
Targum Yonasan: The moment Yitzchak left Gerar, all the wells dried up and all the trees stopped bearing fruit. Realizing that this was because they had driven Yitzchak away, Avimelech came to ask him to pray for them.
What is "Achuzas me'Re'ehu"?
Hadar Zekenim, Targum Onkelos (like R. Yehudah in Bereishis Rabah 64:9): It is a group of his friends.
Hadar Zekenim (like R. Nechemyah, in Bereishis Rabah 64:9): It is a friend named Achuzas.
Why did Avimelech bring "Achuzas me'Re'ehu"?
Ohr ha'Chayim: Avimelech wanted Yitzchak to swear b'Rabim (or Al Da'as Rabim), so he would not be able to permit it. The one who says that Achuzas is a group holds like R. Yitzchak (Gitin 46a), that 'Rabim' is 10. The other opinion (refer to 26:26:151:2) holds like Rav Nachman (46a), that 'Rabim' is three. There were Avimelech, Pichol and Achuzas.
Ha'amek Davar: He brought many friends to appease Yitzchak, if necessary.
QUESTIONS ON RASHI
Rashi writes: "Do not be puzzled by the letter Sav in the word 'Achuzas' [me'Re'ehu] ...." What is difficult in explaining this letter?
Gur Aryeh: Now that Rashi has explained that the letter Mem in "me'Re'ehu" is a prefix, meaning "from his friends," we no longer need the construct form in the word "Achuzas" (which would mean 'a group of from his friends'); the verse should write "Achuzah me'Re'ehu." Rashi answers by citing other instances in which a Sav is added without changing the meaning.