Bridesmaid dresses- do not steam?

Usually you can steam them by putting cheesecloth on the head of the steamer and you don't use tap water. I've never heard of an article of clothing you can't steam

Me too it's so weird, but the tag says do not steam. It's 100% polyester too

You have a while before your wedding so I would be emailing the manufacturer NOW. Ask them exactly what you should do to care for the dresses so you get it straight from the horses mouth. Also, provided your wedding is not a DW then I would have everyone hang the dresses the second they get them in hand. That way as much as possible of the wrinkles will just fall out over time. If they do that and can bring the dresses in garment bags you shouldn't have too much trouble to start with. It'll only be an issue if the dresses have to travel for any reason.

I've never steamed or ironed any dress for a wedding I've been in. Just hung on the hanger several weeks/ months.

Clarify with the manufacturer. But, you may be able to press them with a warm/dry iron. Steam may leave spots.

When I worked in the bridal salon how we steamed dresses was first to turn them inside out and then the steamer had a cloth or nylon over the head of it. Some of the items we used an iron on but those were few and far between.

I was in a wedding with Mori Lee BM dresses that also said Do Not Steam. We had all traveled to California and we steamed them anyway... we just did it carefully but it was fine. They were chiffon.