First threesome

Talk to them what is okay and what is not. Tell them your reservations.
And then take it easy and slow to begin with and play it as it comes.
 
Take your cues from the guy. He is inviting you to join him in pleasuring his gal. Don't ever do anything to put him down or make him think you are going to try to take his gal. You're goals should be be to make her feel very good; make him feel good from how hot his wife is to you; and become a friend with benefits (FWB) who is invited back often.