Should I pray the Sunnah when a guest is present with me during congregational prayer, or should I leave the mosque to wait for him?

If you are able to pray in your home, then pray the regular Sunnah prayers there, for a person's prayer in his home after the obligatory prayer is better. However, if you are unable to pray at home, do not leave the mosque before praying the Ratibah (regular Sunnah prayer) unless there is an urgent need that requires haste. The Ratibah is not obligatory, and it will not cause a significant delay for your guest. The guest should also adhere to the Sunnah.