“sent for Ahithophel.” Ahithophel had been David’s counselor, but apparently he had become angry and bitter against David after David had sex with his granddaughter Bathsheba and arranged for her husband Uriah to be killed. Ahithophel was the father of Eliam (2 Sam. 23:34), and Eliam was the father of Bathsheba (2 Sam. 11:3), making Ahithophel the grandfather of Bathsheba. Absalom would have noticed that the relationship between Ahithophel and David had cooled, and likely had spoken with Ahithophel while they were both at the palace, and that is why he was confident enough to send for Ahithophel even though in the past Ahithophel had been very close to David. Now that David had a rival, Ahithophel sided with him. His anger and bitterness was his undoing, because when Absalom rejected his advice and took instead the advice of Hushai the Arkite (2 Sam. 17:1-14) he felt so rejected and dishonored that he took his own life (2 Sam. 17:23). [For more on Ahithopehl see commentary on 2 Samuel 16:21 and 2 Samuel 17:1].
“from Giloh.” Giloh was close to Bethlehem, to the west of it, and is mentioned in Josh. 15:51.
“and the people increased continually with Absalom.” We are not told why so many people chose to side with Absalom against David. No doubt some thought Absalom would make a better king than David. But it is also possible that some of the people who sided with Absalom at this point thought that David, who was getting older, must have appoionted Absalom to be king. After all, Absalom was David’s third son and the first two were dead, making Absalom the crown prince.