We do this pretty regularly with a guy we know.
It's just sex. He only visits for a sex night. We don't socialize with him at all.
He's the UPS guy at her office, they do occasionally see each other but not often at her work.
It's very obvious my wife likes sex with him but there's no emotional attachment that I can see.
She/we do anticipate his visits, we have a fun night with a LOT of sex for her, when he leaves its over, she and I have some great sex for days after he's been with us. We don't really talk about him other than when we might call him. I'm sure he's in her mind but she's never said anything about him.
When he's with us they do everything a couple does in bed, kiss, long makeout sessions, sex in many positions, she cums with him, he cums in her, they've done anal, she's swallowed his load, he's licked her to orgasm...they do everything her and I do.
We're always all in same room, I'm watching them or he's watching us. It's a comfy, relaxing night of sex for all 3 of us.
When we're done she always walks him downstairs to the door for a goodbye kiss. Some times they go one more round in the livingroom while I'm up in the bedroom and I'm good with that. After he's gone we always fuck a little more but sometimes I'm just too worn out and can't cum. In the morning we always have sex and I'm recharged enough to get the job done. I will say she and I both enjoy that morning sex as we both have the mess from the night befor still on us.
It's just sex and we do enjoy the fun.
99.99% of our fun has been with him, it took quite a few visits to get comfy, was very very awkward first times, lots of guilt and crying the following day but we're over that now.