NET

21 The Hebrews who had earlier gone over to the Philistine side joined the Israelites who were with Saul and Jonathan.

Darby

21 And there were Hebrews with the Philistines^D1831 before that time, who had gone up with them into the camp round about; and they also [turned] to be with the Israelites who were with Saul and Jonathan.

Darby Bible Notes

Philistines - Or 'who belonged, as bondmen, to the Philistines.'

KJV

21 Moreover the Hebrews [that] were with the Philistines before that time, which went up with them into the camp [from the country] round about, even they also [turned] to be with the Israelites that [were] with Saul and Jonathan.