Foursome Rules?

So this guy I've been messing with suggested adding another couple. I've never done anything like this. I'm still thinking about it and seeing if I'm even fit for it. Which is why am I asking if there are any upspoken rules I should be aware of? 
Also, would that mean I'd have to hook up with the girl? Cause...I don't think I'd be able to do that. 
Advice? Tips? I'm seriously clueless on this. 

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