When Muersualt, Marie and Raymond take a bus to Masson's beach house on the outskirts of Algiers, what do they notice a group of?

In Part I of The Stranger, Raymond and his
entourage notice a group of Arabs following them.  One of the Arabs includes the brother of
Raymond's girlfriend, the one he beat.  As such, the Arab men want to take revenge upon Raymond,
but they want to get him alone to do it.  Knowing this, Raymond invites the group as a kind of
protection.  He also wants to lay low out of town, because he knows the police will be asking him
more questions about the abuse.

Meursault is an unwitting accomplice
in Raymond's revenge against his girlfriend; he serves as a witness and writes the letter as
bait.  The excursion to the beach begins the turning point of the novel, and it contrasts with
the earlier beach scenes with Meursault and  Marie at the beginning of the
novel.

In the earlier beach scene, Meursault had lived a carefree
life, but with Raymond, Meursault mistakenly attaches himself to the culture of violence,
revenge, and death.  Raymond's cycle of revenge ironically leads to Meursault's death: Raymond
beats his girlfriend, so she calls the police.  When he is acquitted, her brother vows revenge.
 After the fight on the beach, during which Raymond is stabbed, the Arab achieves a kind of
revenge.  But Meursault, carrying Raymond's gun as protection, has his freedom threatened on the
beach, and so the gun almost fires itself.  It's an absurd ending for
Meursault.

Groups are very important in the novel: the group of
mourners, the group of Arabs, the jury, and the mob at the execution all stand as threats to
Meursault's personal freedom.