They brought him to Jerusalem (2Samuel 19)

The SAB marks 2Samuel chapter 19 as "boring", and yeah, it probably is; but what do you want, more mass killing and rape?

David is still mourning Absalom, and Joab goes to find him and says essentially that it's looking bad that he's mourning someone who wanted him and his family and friends dead, and if he wants to stay king, he needs to buck up and go talk with the people. So David goes and sits by the gate of the city, a traditional place for leaders to be, and the people notice and come to him. David sends word to the tribe of Judah (his own tribe) and asks why they haven't sent for him to come back, and tells Amasa that he will be the head of the army in Joab's place. They send for David, and he goes back to cross the Jordan.

A bunch of people show up at the crossing. Shimei shows up with a thousand Benjamiites and apologizes for the way he treated David on the way out. Mephibosheth shows up and says that he has been slandered by his servant. David is not in a mood for revenge at this moment, so he forgives them. An 80-year-old man named Barzillai, who helped David when he was in exile, shows up to see David back to Jerusalem, at least part of the way. David returns to Jerusalem, and there's a bit of an argument between the men of Judah and the men of the rest of Israel over who is more loyal, which apparently leads to problems in the next chapter.