I just talk about it as part of regular conversation as it becomes relevant. It isn't really relevant to my co-workers, but i'm not afraid of what they think should it ever become relevant. We have Open Relationship on our facebooks, and Spouse told me that their sisters were giving them "concerned" looks at Thanksgiving and asking if "everything is ok between you and BG" (because i was spending tg with a friend who had nowhere to go this year.. last year, whatever). I guess to a lot of people, "open relationship" is something you do when you are breaking up but still get along well enough to share a living space... Well we just celebrated our 10th/13th anniversary.
Anyway it doesn't have to be a big huge "hey sit down there's something i need to tell you" event unless you make it one.