Couples dating couples
One you've all decided that you're in, the couple are likely to have a few rules about the way they want their relationship with you to work. And what if they want to see people other than you? Sure, sometimes you won't know until you try it, but if the idea of having serious feelings for multiple people causes a panic attack, emphasise that you need to keep this light and fun.
One of the beautiful things about being close to an existing couple is that they will have established patterns; integrate yourself into them as organically as you can. If not, give them space to just be a couple when they want it.
Over time, this can evolve into a triad situation, in which every party is equally committed to each other.
But this is rarer and, if you're just starting out with this couple, it's unlikely to be what they're proposing. Whether you're looking for casual fun or actual inclusion in their lives, you need to establish your expectations early. Can you happily fall in love with more than one person? If this couple is keen to get emotionally intimate, you need to know if that's something you can handle.
Do they want to keep the relationship on the down low? Figure out what you'll want and need from them and be upfront about it. So there's a chance this couple will teach you some new, fun tricks.
Do they want it to be mostly about sex with very little "cooking pasta together" domestic bliss? But if there are certain items that are permanently off your menu, let them know.