Adam Clarke on Acts 21:18

Went in with us unto James - This was James the Less, son of Mary; and cousin to our Lord. He appears to have been bishop of the Church in Jerusalem, and perhaps the only apostle who continued in that city. We have already seen what a very important character he sustained in the council. See Act 15:13.

All the elders were present - It appears that they had been convened about matters of serious and important moment; and some think it was relative to Paul himself, of whose arrival they had heard, and well knew how many of those that believed were disaffected towards him.