I know very few people agree with me, but I'm of the opinion that you should always allow your guests to bring a date. From experience as both a planner and a guest, there is nothing more frustrating than milling around a party without a date when most people are coupled up. Also, there is nothing that reminds a single person more of their single status than the celebration and union of marriage. With that being said, that's just a personal opinion and not necessarily etiquette. However, what is etiquette is if they are married, then yes, yes, yes, you need to invite their spouse. If you don't have the budget, consider not inviting the coworkers at all rather than inviting them sans the most important person in their life.