Swinging with good friends will either make your friendship that much stronger, or will kill it. Simple as that.
How you and they go about it is what will most likely determine which way things will go.
As Cyber said, before you do anything else. Talk with your mate and make sure that this is something that both of YOU want to do. You didn't mention if this was a case where both you and the other couple are experienced swingers or if one or the other is just starting out. That can make a difference here.
Make sure that any issues are out in the open , as well as any ground rules. If there is something you aren't comfortable with make it known. You are all already good friends so being able to have a frank and open discussion about swinging and any issues you have should be easy.
Once you are all sure that this is a step you want to take together, it's up to you to decide how best to do that, based on what you are all comfortable with.
When it's all over things might be a little awkward at first but help each other get over that by talking about the experience together and how you felt about it. If there was any jealousy issues or any other issues that cropped up during the fun, get them out in the open so that no bad feelings can grow and hurt your friendship.
As long as you treat this as an extension of your friendship and are open and honest with each other you should all be fine
Julie
http://www.swingersboard.com