Just friends in dresses they already have as bridesmaids?

What you are suggesting is what is traditional. The notion of bridesmaids as a sort of chorus line in identical dresses is a very recent innovation and doesn't seem to serve any purpose other than creating stress and expense for all involved and enriching vendors of such clothing. So long as you have some harmony for a pretty visual, and don't have one lady in a formal and one in a tail dress and a third in a business suit, I think it will be splendid.