This is a touchy subject and there is no right or wrong answer. It all depends on the dynamic of the individuals involved. I've had a lot of threesomes and it was almost always with friends (which turned out the best). It also depends on what you're looking for. Do you want just a little sex on the side, without her being involved in any way, not even hearing about it? Then it's probably best to find someone that neither of you know. If it's more of a group activity where she'll feel included, she might be more inclined to know it's with someone you all care about and know well, but ultimately, that depends on the person in question.

Personally, I would try to find someone that neither of you know. I would suggest that first and foremost, otherwise it might look like you just wanted a good excuse to sleep with her friend, rather than the fact that you're serious about this as a lifestyle.

Perhaps start by bringing up the discussion and asking what's off limits. If she tells you friends are off limits, well, there you go. If she doesn't, let her speak and say everything she needs to without judgment, then ask when appropriate.

Also, don't worry about what the community thinks is right or inappropriate--do what's right for you and your wife.