A question for couples - introducing a 3rd person

I have been wondering about the situation for some considerable time. I have yet to become involved with a couple and was just wondering if thise that have would share there experience. how did / do you go about selecting the right person? What were the next steps taken? Where you all involved right from the get go or did you take things slowly by meering your guest alone to start with? I would dearly love to become involved with a couple and hope this aids in my quest. can't wait for your replies.