Vixen and PD, this essay be of some use to you: http://www.morethantwo.com/coupledating.html
It has some great advice for situations in which one person is getting involved with a couple.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.