How to be polite

Unless your venue has a dress code (eg men must be wearing jacket and tie to gain entry), unfortunately there’s no polite way to enforce a dress code. Honestly, I have very little recollection of what anyone wore at our wedding - you’ll be so busy and so happy chances are you won’t notice! I do understand how you feel though - I know someone who has turned up at a black tie event in jeans and a floral shirt and honestly, the amount of side-eye he got from other guests, me included, was insane.

All you can do is say your dress code is cocktail and pray people comply. If they do not though just move on some people are selfish.

I think it depends on how close you are. We have a close family friend that due to health reasons can’t really go out and buy something nice. My mom was worried about it so she sent her a dress. The friend was very thankful. If you’re close to these people maybe have a private conversation and ask what they plan on wearing. But ultimately you can’t force another adult to dress a certain way so be prepared to let it go.

We were worried about people showing up in jeans and didn’t want them to.
I posted something in the FAQ of our website that requested no jeans. I also included something on the details card in the invitation. But worded it as since our ceremony is in a house of worship please wear respectful “church clothes” I don’t remember my exact wording, but no one wore jeans.

I would just say it's formal attire and specify no jeans. You will get people that ask what you mean by formal because there are differing opinions on what people think is formal attire. I've had people say formal attire on their invites, only to find out they just didn't want anyone wearing jeans. We are having a legit formal wedding, so no jeans and no cocktail dresses. Everyone on our guest list already owns suits and correct length dresses, so it's a know your crowd choice we made.

I agree with pp's that except for making a suggestion on your website, you should let it go and let people decide for themselves how they'll dress. Daughter and SIL's wedding was "semi-formal" at a very nice country club-type venue, and the vast majority of guests dressed appropriately. There was one guy who showed up in jeans and a couple of women who showed up in pretty casual pants/tops (they were all middle-aged friends of the FOG, who was in a black suit...). They kind of stuck out as not being dressed appropriately, but it didn't really matter. Daughter's photos are stunning, and even those the "under-dressed" are in, it isn't noticeable what they're wearing -- they blend into the crowd.

On your invitation you can write like black tie optional so people know to dress formally

Unfortunately, there’s no polite way to say “you typically look like you don’t brush your hair or wash your clothes, can you look better for my wedding?” Most adults know to look nice for weddings and will do their best. I assure you they don’t want people talking about how awful they look to each other or online forums. Your invitation and venue should indicate the formality of the event and your guests should do their best to match the occasion. Outside of that, it’s out of your control.

You could say on your invites "cocktail attire reception to follow" when referring to the event. This to me would signal, hey I need to dress up. A lot of the time, people don't know what to wear. It can be confusing because you never really know what the venue will be like or the vibe.

View Quoted Comment That's a great idea, thanks!

I included "cocktail attire" on my website but the reality is, you can tell them all day long and some people are gonna wear whatever they want. Unless you are hiring security or your venue has a dress code policy that someone will be around to enforce there is really no way to control it. There's also a good chance that you'll be so busy with other things and so swept up in the night that you won't even notice those particular few.

View Quoted Comment

I totally agree with this suggestion. A lot of time, I ask the bride specifically what she wants guests to wear because "semi-formal" can have a broad range of meanings. It's been very helpful that the last 2 weddings I went to had an FAQ page which addressed dress code. I would do that and provide specifics there such as "dress shirt and slacks for men and sundresses for women," or whatever your ideal is to make it clear what you want. Good luck!