Shepherd the flock of God which is among you, exercising the oversight, not under compulsion, but voluntarily, not for dishonest gain, but willingly;
So Silas, who was the companion of Paul on many of Paul's missionary journeys, is now a companion of Peter. Perhaps Paul is in prison at this time and so Silas has gone with Peter, and he is the one who writes, does the actual writing of this epistle that was dictated to him by Peter. And Silas was known to a lot of these people because he had traveled with Paul. Peter had not known many of these people, but Silas, having been around with Paul, he's "a faithful brother unto you, as I suppose, and I have written briefly,"