I answered this question before in another forum, and had received some flack for it. But realistically, when you do this for the first time, you may already have a scenario of how it will go in your mind. It can be quite a different experience actually participating, regardless as to how everyone thinks they are on the same page.

I have done this a couple of times, one time was spontaneous, and the other was planned out. Both were equally satisfying and fun.

If you want to try and experience things such as this, talk a lot first to your boyfriend, and figure out what you both want.
Do you want to be with another woman, or with two men? Do you like the idea of your boyfriend being with another woman?

Think about things as well, jealousy may pop up in the middle of your play even if you thought otherwise.

Always remember who your boyfriend is and he should do the same. Getting into the heat of the moment with someone else may cause friction between the two of you.

Getting to know the other people is good as well, feeling comfortable, and knowing what their expectations are makes the experience more enjoyable for everyone. Although sometimes spontenaity is a good thing too

My six cents on the subject, and it is the philosophy that I follow.
Hope that some of it helped a bit.

I realized that I didn't really answer your questions, but you really have to know what you want and why you want it.
Both of my experiences were good, I would definately do it again, and I wouldn't change things. But doing that is kind of my kink, and I don't ponder things at all. If I had an inkling of doubt or regret I would probably reconsider ever doing it again.