First piece of advice is not to over thing the meeting to much. You will set the mood for the meeting, and if you are thinking of everything that can go wrong, then that may set an uncomfortable mood. Just be positive. Let it happen as it happens.
My second piece of advice is that a short first meeting is better than a long one. For example, when my wife and my girlfriend met for the first time it was a a local coffee house over a cup of coffee. This had two advantages. The first was that it was in a public place. Neither of my lovers felt that they were intruding on the others private space. The second, was that it was a short meeting (10 - 15 minutes). This was long enough for them to meet and get to know each other, and sort enough to not overwhelm either of them
My third and final piece of advice, as others have also said, is that this is a meeting. Romance, kissing and other forms of affection can make one or both of them uncomfortable. While these are hallmarks of Polyamory, they should be avoided on a first meet.
I hope that you meet meeting of you husband and your lover goes well.