I was praying the Sunnah before the iqamah, and a person joined me [in prayer]; what should I do?

It is obligatory for you to complete your prayer according to your intention. Once you have finished the Sunnah prayer and given the salaam, your companion should complete what he missed. The preponderant and authoritative opinion we hold is that it is permissible for someone performing an obligatory prayer to be led by someone performing a supererogatory prayer, or vice-versa. The matter is even easier if the follower is performing a supererogatory prayer.