Welcome to the boards.
If you're in the habit of speaking freely about all subjects, just begin talking. I view additional relationships as expanding my life and making for more good things for me and mine, so I'd speak of it in those terms--nobody's losing anything and instead gaining a good deal.
I'll also suggest sorting out exactly what it is the two of you require from a relationship for it to be worthwhile and discussing how to make certain you provide that for each other while adding other people to the mix. That goes a long way to addressing fears of loss.