I'll attempt to answer your question but I am by no means an expert. Others who may be more qualified to answer will hopefully come along to add to or correct what I say here.
Your best bet is probably at some kind of swingers/sex club or event. Some members have said that they have had success approaching men in public, but that could be weird or awkward. At least if you are in a swingers club, everybody pretty much knows why they are there and the atmosphere is conducive for it.
As for what the husband does, as much or as little as the two of you decide. Some people might respond and say that it's up to the "bull" what role the husband plays, but you are married to your husband, not the third person in the arrangement, and the two of you should be deciding what role your husband plays (it should go without saying that this should be well planned out and all these details worked out in advance, and open, honest communication is vital). Some couple play together, in other words inviting a third person into their bedroom is going to result in a 3some, while in other cases, the husband might watch, take pictures or video, or in some cases the husband may just be their for the initial meeting then retire to a different room while the action unfolds. In whatever scenario you and your husband decide to place yourselves in, make sure that you do not compromise your rules, i.e. if the two of you set ground rules and the third person decides that he doesn't like them, then move on until you find the person is amenable to your rules. This is meant to enhance a relationship, not divide it.
