So a few words about partnered or group sex magick....
Many of us experience the magickal energies as sexual in nature. They hit our radars as "I am attracted to you..." Sometimes that can be a great thing and sometimes that can be WAY more trouble than it is worth. We are all adults, so buyer beware :)
Also, sex magick works no matter what set of plumbing is involved. It is not about the plumbing it is about raising energy and directing it to a target. Just make sure that they are informed participants. No fair springing sigils on the unknowing or unwilling.
Please keep in mind that this process is not just sex for pleasure. There is a distinct process of energy sharing. So if you do not want to get this persons astral goo do not, I repeat, do not engage in sex magick with them. You will share energy and that almost always directly equates to issues. If they are bugfuck crazy ( to steal a line from Von Faust) then you will get a really good dose of that too. Crazy is a communicable disease, just keep that in mind.
So now that we have gone through the preliminaries there are a few things that a group dynamic changes. Before the process starts you need to work out who is running the show and who is along for the ride. Once you have that established I recommend raising the energies in any way that works for you. Once you get things flowing, then take a face to face position. Work on synchronising your breaths since there is nothing more basic in getting energy moving than breathing.
Slowly start to envision your chakras expanding and joining. If you have colors in your paradigm great if not just work with them as whatever colors seem appropriate. Some people have particular colors hardwired in and others do not. Just do not let it become a distraction.
Start at the feet and move your way to the crown. If you can keep eye contact through out the process you will find that it is easier to sense the chakras and energy movement. One you can feel things moving in a rythmn and that you are connected you can start to push the energy around to balance it. It functions similarly to a middle pillar. When you are circulating the energy, sometimes there will be a tingling sensation at the base of your spine and it will move up to your crown. You are not dying (despite my first instincts when I encountered this...) it is a function of the energy awaking the kundalini. Good stuff in my humble opinion.
Now this is not a marathon session, unless you want it to be. There is no energy o'meter that you can plug in next to the bed. At a certain point you have raised enough energy to charge your talisman and you follow the same procedure as you do by yourself. At the moment of orgasm you focus your intent on the sigil. Afterwards, move your focus back to your partner. You have just been connected very deeply and you want to disconnect - gently. Reverse the process and make sure everyones chakras are whole and closed.
If you chose the charged fluids can be used to charge the talisman or sigil also. If you are squimish about the concept of charged fluids, you might want to consider a different magickal path :)
While I suspect 3 people will actually read any of this for magickal purposes, I hope to have done right by you :)
Good luck and may all of your adventures be universally benifical.