For what is our hope, or joy, or crown of rejoicing? Isn't it even you, before our Lord Jesus{TR adds "Christ"} at his coming?
His coming -- Paul would have joy when he saw the Thessalonian Christians in the presence of Christ at His coming (at the judgement or in heaven. This is a good answer to the question, "Will we know each other in heaven?" Paul indicates he would know, or recognize, them.)