The offending tribe, Benjamin, to which the men of Gibeah belonged, was not present. In fact, they are presented as outsiders: they heard
about the summons to Mizpah and that the people of Israel had gone there. It almost reads as though this information came to them second-hand. In other words, they were not invited.
3 (Now the people of Benjamin heard that the people of Israel had gone up to Mizpah.) And the people of Israel said, “Tell us, how did this evil happen?”