We approached this a little differently. It was the wife's birthday and we happened to be going to a non-swing event where several of our swinging friends were also in attendance, including some single males.
The event was beiing run by a couple we knew well who also swing. We stayed to clean up and then one of our friends invited us over to celebrate the wife's birthday, along with another of our mfm partners and the couple running the event. Well, the wife and the two guys headed up to a bedroom, where I joined them with the husband of the couple. I was nearly an mmfmm, but the wife of the other couple grabbed her husband and when we heard the bedsprings squeaking in the other bedroom, we figured he wouldn't be joining us anytime soon.
Anyway, it turned out to be a great time for all. She loved it and expressed a definite interest in doing more than three in the future. Basically, for us, we have enough partners to do this without having to ask one of the guys to bring anyone new into the situation. For the wife, she finds it much more relaxing to do this with guys with whom she's already been.
We'd feel like we'd be putting unnecessary pressure on the guy and introducing a factor that could possibly ruin the good relationship we already have. Just our two cents.