I got picked as second tier bridesmaid. And what that boiled down to was that in the event that any of the other bridesmaids couldn't fulfill their role I was jump in.
What it turned out to be was me folding, stuffing, and addressing tons of wedding invitations for a rustic wedding.
Personally, I am not a fan of cutesy poems in wedding literature. And technically, suggested dress is not traditionally mentioned on an invitation. That said, no one knows that because everyone does it anyway. But I had to grimace just a little while I was doing the aforementioned task. Why? They stipulated what was to be worn.
I decided, rather than complaining, I would simply addressed the matter on my website.
Regarding Dress Code
Be careful with telling guests how to dress, especially casual. For example, I want to call my wedding relaxing and casual, but I mean this in the sense of not super structured, plated dinner, formal dancing time, etc. But I wouldn't tell my guests that my wedding is casual, because that could means jeans or shorts to them, and show up whenever you want ordeal.
But if you don't mind that, then go for it! Honestly, your guests have been to plenty of weddings before and they don't need attire advice. I would see that your wedding is at a rustic barn and outdoors, and know not to wear a cocktail dress, but a cute summer dress with sandals.
All In All
I had fun. It was some work, but I didn't expect to be in the wedding. Not really, and I got to spend an afternoon or two helping.
It was easy since we took the earliest date that vendors needed to know and put a RSVP due date that was 2 weeks before the earliest vendor due date. So with your RSVPs, you need to know answers 6 weeks before.
This is the timeline we had. Wedding was end of Dec, one vendor needed final count 1 month in advance so end of Nov. We made RSVP due date be early Oct giving us a 2 week cushion. We sent out invites end of Aug/ early Sept giving folks 1 month to respond.
Then the 2 weeks after the due date and before the vendor due date was spent chasing/calling folks who had not responded.
Seems reasonable to me, but I don't know a lot of the invitation etiquette.