Old School Rule #2: You must invite everyone with a guest.
New Rule: If they'll know others, skip the plus-one. It's still polite (and very appreciated!) to invite guests' significant others, but if you're inviting a group of coworkers, for instance, and two or more of them are single, they should have no problem attending solo. Only when guests won't know anyone aside from the couple is it mandatory to let them bring a date. It's kind to invite attendants with guests too (they are shelling out big bucks for their attire!).
Photo: Shira Weinberger Photography / The Knot