Jamieson, Fausset and Brown on Philemon 1:23

23. The same persons send salutations in the accompanying Epistle, except that "Jesus Justus" is not mentioned here.

Epaphras, my fellow prisoner--He had been sent by the Colossian Church to inquire after, and minister to, Paul, and possibly was cast into prison by the Roman authorities on suspicion. However, he is not mentioned as a prisoner in Col 4:12, so that "fellow prisoner" here may mean merely one who was a faithful companion to Paul in his imprisonment, and by his society put himself in the position of a prisoner. So also "Aristarchus, my fellow prisoner," Col 4:10, may mean. Benson conjectures the meaning to be that on some former occasion these two were Paul's "fellow prisoners," not at the time.