Adam Clarke on Galatians 1:2

And all the brethren which are with me - It is very likely that this refers to those who were his assistants in preaching the Gospel, and not to any private members of the Church.

Churches of Galatia - Galatia was a region or province of Asia Minor; there was neither city nor town of this name. See the preface. But as, in this province, St. Paul had planted several Churches, he directs the epistle to the whole of them; for it seems they were all pretty nearly in the same state, and needed the same instructions.