Groomswomen Attire

We had a groomswoman, my husband and his other GMs were wearing tuxes, so we told her she could either wear a tux with them, or a long black dress (BMs were in burgundy dresses and she chose a long black dress). I like your idea of jeans and a nice dress shirt (maybe red?) you could also do an ivory dress shirt with maybe a red maxi skirt?

My FH has a Groomswoman and I asked her if what she would like to wear - a suit or a dress. She ultimately chose a dress that's the same color as my bridesmaid dress, but in a different style. You could also do a skirt and blouse since the guys are wearing jeans.

My first thought is a dress similar to this. Perhaps an ivory top and a skirt with red or a dark wash bottom. That way she's not completely in an ivory dress.

Have you considered having her wear the same color bridesmaid dress but just a different style? This could be a good option. Your wedding sounds laid back, so dark wash jeans may look nice then a nice dress shirt to match the men. Will she get a bouquet (if your bridesmaids have them)?