What should "groomsmaid" wear?

I think you should put her in a grey dress that is the same style as the other girls.

My FH has a "Best Ma'am" and we're going to put her in a long dress to match the formality of the other groomsmen in their suits/tuxes, and to differ her from my BM's in short dresses. We haven't picked out dresses for my BM's yet, but we'll keep the Best Ma'am dress more formal cut than the BM dresses... for example, no one-shoulder flower/ruffle/bow accents on the Best Ma'am dress, but maybe on the BM's dresses.

I'd agree with Pumpkin and think you should go with a long, grey dress.

I'm with Pumpkin's Sunshine. I'd definitely have her match the groomsmen rather than the bridesmaids. Actually, I'd be inclined to have the bridesman in a navy tux, but it's obviously up to you.

I wanted her to be in the same dress but in grey, but it doesn't come in grey. :/ It's an outdoor summer wedding, so long dresses are out.

What do you guys think of this combo:

BMs in this dress, in 'Marine' color: http://www.davidsbridal.com/Product_Short-Satin-Off-the-Shoulder-A-Line-with-Ruching-83168_Bridal-Party-Bridesmaids-Shop-By-Color

Groomsmaid in this dress, in 'Pewter' color: http://www.davidsbridal.com/Product_Short-Sleevess-Satin-Dress-with-Ruched-Waist-F14823_Bridal-Party-Bridesmaids-Shop-By-Color

I would stay put her in the same dress since the bridesman is in the same tux color as the rest of the guys.

Those are very close. I like it.

We are having a groomsmaid and a bridesman and the groomsmaid (as well as all of the bridesmaids are wearing the same color but in different styles) and the bridesman is wearing the same colors as our groomsmen.

You could always put her in a dressy pants suit that kind of matches the groomsmen

I have a groomsmaid too the way we did it is that she is wearing the same dress as my BMs but it is black to mach the GMs suits

My BIL's Groomsmaid wore a black dress to match the guys with a matching sash that matched our dresses (kiwi).