Right now, I am in a poly relationship with a couple. I talked with them at lengths before coming into the relationship and committing to them. For me, it isn't the fact of being in a poly relationship as much as it is about having more than one Owner. I am a slave to both of them but I am also helping Her out by helping with the animals, household things, etc. I am also her friend that she can share things with. He is the Master where she is the switch. Both of them have used me just as I have pleased both of them.
I can honestly say that this style isn't for everyone and when I first started looking I really wasn't looking for a couple but over time I realized that there are things that He enjoys but she doesn't like doing where I do. An example is anal. She doesn't care for it where I do. Needles is something that she is terrified off but I am not.
I think the best thing to do is make sure that whoever you are bringing in doesn't get jealous because it can destroy the relationship. Communication is key in any type of relationship this being one of them. I have found that I can talk to her easily than I can Him.
Good luck.