You can find several couples having sex on webcam and swapping sex partners on the websites below.
Chaturbate allows you to visit public chat rooms for free and watch group sex shows live. Then, you can join Chaturbate and send tips to see the sex shows you want. You can also get in a private webcam chat where the group will be able to see and hear you. Note that those cam models are amateurs.
Stripchat is very similar to Chaturbate, it allows you getting in chat rooms of cam models for free. Then, you can tip them to watch a group sex show live on webcam. You can also get in a private chat room where you will be able to interact with the group in a optional 2-way audio/video webcam chat. Most of the cam models come from Latin America and Central/east Europe. Be aware of the time difference.