i think guests really appreciate when there IS a dress code stated on the invitation. especially since proper etiquette isn't common knowledge these days.
i found this site from googling...

Guide to Wedding Dress from DressCodeGuide.com

i like "cocktail attire" or "evening attire" better than "semi-formal" etc. but that's just my word preference.

once you provide them with the dress code it's up to them to dress appropriately for the season.