Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 139
Excellence of Greeting on Arrival and Departure

869. Abu Hurairah (May Allah be pleased with him) reported: Messenger of Allah (PBUH) said, "When one of you arrives in a gathering, he should offer Salam to those who are already there, and he should also do so when he intends to depart. The first act of greeting is not more meritorious than the last.''
[Abu Dawud].

Commentary:  We should offer As-Salam to those sitting in a gathering, on our arrival as well as departure. Both the greetings are essential. "The first act of greeting is not more meritorious than the last'' means that we should utter it on both occasions.