What is some advice on joining another couple in sex?

me and my husband got ask to join another couple in a foursome... we know the couple but don't know them in that way... we are pretty sure everything is a go with them as far as the medical history is concern. In a weird way i kinda want too because it would be something that i have never experience and i kinda like the wild idea of it... my husband has some concerns but he is still trying to wrap his head around the idea. tell me what do you guys think?