How to initiate a foursome ?

Me and my boyfriend overhead my roommate having s3x with her boyfriend. The guy fcuked her damn fast and I got crazy for him ever since, and my roommate moaned awesomely !! and my boyfriend wants her ever since !! :) these days when me and my boyfriend fcuk, we imagine my roomie and her boyfriend doing with us and get totally turned on. Now we seriously want to do it with them in real. But we are not sure if they would be ok with it. Please suggest ways on how to ask them in a way that it doesn't spoil our friendship or suggest ways to attract both of them into this.