I would suggest reading other threads first.
Are links to lots of great ones. You may also want to really stop and think about what you are looking for and how it will work. NOT just for you two, but a third person who is a human being and if nothing else, put yourself in their position. Too many times a couple comes on thinking a third would be great. From your POV it's all, "Hey we are a great couple, we'd treat her great, it would be all warm fuzzies!"
Now look at it as a single woman being approached to be brought into an existing relationship. "They are the couple, I'm just there when they want me and am constantly unsure of my position as it is just as a third wheel to THEIR relationship."
It's common for couples to think this is the easiest and best way to advance. While it's entirely possible for three to be in a relationship you need to do much much more reading on poly and consider how what you are saying will come across to a stranger. Reading here will show you most people, not at all happy about being asked to be 'brought into' an existing relationship. Most of us, kind of looking for our own relationships. Not to be a fun add on to someone else's.