por the time Harry and Ron went into the forest, Hagrid was already in Azkaban, for a crime he never commited. The boys took Fang with them when they went to meet Aragog, who almost killed them. Hagrid never went into the forest in either book o movie.
If tu are talking about the segundo movie, it was Ron and Harry, but only following Hagrid's consejos (he had already been taken to Azkaban and was unable to accompany them). If tu meant the first movie, it was Harry, Ron, Hermione, and Draco (in the books, it was Harry, NEVILLE, Hermione, and Draco; Ron had not been dado detention).