Vogue recently did a fantastic article on Plus-One Wedding Etiquette - here are some of our favorite bits from the article.
As a rule of thumb, only married, engaged, and “serious” couples (say, they’re living together or have been together for a year or more) receive a plus-one.
For the wedding party, rules don’t apply. The rules about cohabitation, dating, and marriage go out the window for your wedding party. Not only does a happy wedding party make a happy couple, but allowing a bridesmaid to bring her new boyfriend, for example, is a small token of appreciation you can offer in exchange for her efforts and support.
Make it easy for singles. Draft a seating plan that fosters a comfortable dynamic for solo guests. For singles, there are fewer things more awkward than being sandwiched between an old married couple or a PDA-heavy pair. But creating a “singles-only” table could give off the impression you’re corralling your single pals. Instead, place them between outgoing and friendly couples who they’ll likely get along with. That’s sure to create a more communal feel to the event, and it’ll help them meet people organically.