Radak: It is part of Yisrael, for verse 42 says that all of Yisrael came [and complained why they did not get to bring him across]! It was only Bnei Yehudah who fled with David, and the 1,000 who came with Shim'i. Chatzi, Mechetzah and Machatzis merely mean part. Usually they are equal parts (it is half); sometimes it is not equal, e.g. "Chetzyo Sarafti", "Ad Tzavar Yechetzeh" (Yeshayah 44:19, 30:28), even though [from the feet until the neck is more than half the body;] above the neck is a small minority!

Malbim: It is half of the 1000 who came from Yisrael [to return the king]. The other half delayed and crossed afterwards.