He may have been the church father known as Clement I or Clement of Rome. Paul asked for his help as well in resolving the women's disagreement. We don't know anything more specific about him. Paul wanted all the Philippian believers to experience a rich fellowship. But to achieve such harmony, each one needed to be a peacemaker by helping settle any discord.
Love is the one word summarizing every believer's duty.
God loves us and we need to love one another. It's the bond that will keep the church stable. Many believers allow themselves to be victimized by their circumstances and consequently vacillate between a spiritual high and low. For them, a command to rejoice seems unreasonable. We can't always rejoice in our circumstances or other people because both can be bad.