First, you must have Lady666 or Amra72 animations. Second you need "Passion" mod or "Kinky world" mod. Third, once you have those, when start having sex click on any Sim you desire and using Passion or  KW mod, find the line: "Join" or "ask to join".  Have you seen this threads?

http://www.loverslab.com/topic/43249-sims3wip-kinkyworld-v028-updated-15-sep-2016/

http://www.loverslab.com/forum/55-downloads-the-sims-3/

http://www.loverslab.com/forum/75-the-sims-3-general-discussion/

http://www.loverslab.com/forum/78-the-sims-3-technical-support/

I warmly recommend to visit those threads and if you don't fins your answer, post your questions there. Someone will reply to you more specifically. 

thank you