How two people are praying together does one of them stand slightly behind. And if someone joins you while you are praying a Sunnah prayer and you become the imam do you pray aloud.

When two people pray together, they stand next to each other, so the imam is on the left and the ma'mum is on the right. If someone joins another person in sunnah prayer, he can choose to recite out loud or not; both are correct. However, they should act according to what the sunnah is. As the ulema mentioned: 1. The sunnah of silent prayers is to remain silent. 2. The sunnah of loud prayers is to be loud. Regardless of whether he is reciting silently or out loud, he says "Allahu Akbar" out loud.