There are some unknowns when asking friends to play with you and if the conversation has never been brought up then I would advise against broaching that topic. BUT we are sexual beings and sometimes the curiosity gets the best of us.
I wanted to discuss another approach that MIGHT help break the ice IF you do choose to move forward.
There are many ways for you to bring up the topic without fully coming right out and asking. For example, have them over, suggest playing a sexy game and see how they respond. If they are all for the game then play it and keep it simple. IF things progress, you are all naked or turned on, maybe suggest having sex side by side, no touching other partners, just enjoying the show. After everyone has finished then maybe talk about how it was with the friends and see where there head is at with the experience. I wouldn't push the play or overstep any perceived boundaries by initiating the touching if that occurs but if it happens I wouldn't shy away from it. I highly recommend that you continue to check in as the night progresses ensuring that everyone is comfortable with the sexy game, the side-by-side play, etc.
WITH ALL OF THIS SAID, be prepared for the repercussions that might present, such as, them pulling away from you, them not talking to you again, etc. These are always delicate situations to maneuver if you choose to go down that road with them.