If you want to have a dress code you will probably have to say so. It could be quite simple "No jeans, shorts or sneakers please!" or a quick request "Smart (casual) dress please." The invitation itself can be quite plain - it sounds like you don't want any jokes about "The big one" etc, so just a casual mention that it's a 50th birthday is fine. And RSVPS are more likely to be used if there are deadlines, and also alternative ways of replying such as phone and email.
You can find some examples of invitation wording here. Ignore the comic ones, some are very straightforward.
You can find some examples of invitation wording here. Ignore the comic ones, some are very straightforward.
