“the army.” The Hebrew text is just “the people,” but in this context, the people being referred to are the army of Israel (cp. Judges 20:31).
“went up and came to Bethel, and wept.” Even though God told Israel to go against Benjamin, they were unsuccessful twice. The fact that Israel wept, and sat, and fasted, and offered offerings indicates that Israel had some sin in their camp, too, and that prevented them from having a quick, decisive victory.
“and sat there before Yahweh.” The people sat before whatever represented Yahweh, in this case the Ark of the Covenant (Judges 20:27) and therefore the Tabernacle was there at Bethel at that time.