I would avoid anything with a narrow heel as they will sink into the grass. A wedge or flat would be best. You can be these little plastic covers to go over thin heels, but I think they look silly and don't always fit or prevent sinking.
I'm not sure if you are asking your bridal party to coordinate shoes, but if you are, please reconsider. No one looks at the shoes anyway. If you are just asking for advice in case a maid has a question about what shoes to wear that they already own - great for being thoughtful and wanting to ensure nobody sinks!
My ceremony will take place in grass and the reception under a tent with a floor. What type of shoes would you wear.....wedge, flats, kitten heel or go all out with 4in heels? Really afraid of the bridal party sinking or getting stuck as they walk.