Groomswoman

Another factor - what does your fiance want? She's part of his party and presumably he decided their dress code. Does he have a preference on how his team should look standing next to him?

I would lean toward a similar style dress to the bridesmaids but in the same colour as the suits. But I'd also ask her what she would be most comfortable in since she's going to be the odd one out.

I'm going to be a groomswoman in my best friend's wedding and I'm basically wearing the same thing as the groomsmen but instead of pants I'll be wearing a knee length navy blue (same colour as the men's suits) skirt, with the white dress shirt and navy blue jacket, with a blush pink tie. I also told them I'd be totally fine with pants if they wanted me to totally match the men, but since they let me choose I went with the skirt since it'll be a summer wedding. I really appreciated that they let me choose what I preferred.

I would ask her. Maybe she will want to blend in with the guys, maybe she will want to stand out a little.

If she's a groomswoman (as in chosen by the groom to stand next to him), I would think she should coordinate with the other groomsmen, not your bridesmaids. She can wear a grey dress, a gray top and skirt combo, a women's blazer and suit pant in grey etc. It'll all depend on the formality of your event.