Hanuman said, ‘And after his wife was carried away, that descendant of Raghu, while searching with his brother for his queen, met, on the summit of that mountain, with Sugriva, chief of the monkeys. Then a friendship was contracted between him and the high-souled Raghava. And the latter, having slain Vali installed Sugriva in the kingdom. And having obtained the kingdom, Sugriva sent forth monkeys by hundreds and by thousands in search of Sita.

In plain words: While searching for Sita, Rama and his brother met Sugriva and became friends. Rama killed Vali and made Sugriva king. Sugriva then sent thousands of monkeys to search for Sita.