When my wife and I invite Carl in on a MFM, she treats him no different than she does me. Carl's my best friend and my wife liked him since they were introduced years ago.
In a 3 some the wife usually has some one-on-one with the other male to get things going.
Here Carl is hold her hair back while she sucks his cock. She holds his nut sack to gage when he's ready to cum. If he says, "I'm ready" (to cum) and my wife moans, that means she'll keep sucking so he can cum in her mouth. If not, she'll quickly mount his hard dick and she'll begin fucking him.
It's exciting for me because she knows his balls are full of cum and she's coaxing the sperm to come out. It's like she's working hard to get the other mans sperm in her mouth or in her pussy.