Need relationship advice

So.. I'm currently in a throuple, I've been with my primary for going on 5 years, and we've been with our gf for a year. As of recently me and my primary have both taken notice in a friend of hers, she's also poly, single, beautiful, and fun to be around, we'd love to explore bringing her into our polycule, but we aren't sure how to go about bringing it up in conversation to her, or our other partner. Wwyd?

We've all gone out together on Halloween, and we'll be going out again to celebrate our partner's birthday next week, so we have all hung out together before.