Hi!
The option to be with Allison or Sophia during the orgy depends on you actions before. Sophia only shows up if she likes you. So during your encounters with her before you need to get brownie points. e.g. chosing Sophia during the visit to the blue room with Kath and Sophia. Not insulting her, accepting her apology, should be self-explaining. If you have enough points with her, Sophia will show up at your chair and start a conversation. When you need to choose an action, follow her.
Btw. never take drugs like the coke during the the orgy, Sophia dislikes drug use with stuff not made by her. (Also dislikes abuses of her own inventions)
Discrepancy already posted help points for Alli