Traditionally, only married and engaged guests received a plus-one to a wedding. Nowadays, however, the rules aren't as strict, and couples tend to be more lenient with their guest lists. But what if you do receive a plus-one and don't have anybody to bring? Do you need to find a date for the night, or is it acceptable for you to attend solo?
Maybe you're not in a committed relationship or your significant other can't attend the festivities. Perhaps you've recently started a relationship and don't feel comfortable inviting your new boyfriend or girlfriend to the wedding. In any of these cases, it's perfectly acceptable to RSVP as a single guest. The bride and groom won't mind that you're attending on your own; in fact, it may allow them to invite an acquaintance that didn't initially make their list. Responding that'll you be attending alone is also important so that the couple can plan seating arrangements, meals, name cards, and other elements of the wedding.
What if you really don't want to attend the wedding alone, but you don't have a partner to bring along? In some cases, it may be acceptable to bring a friend instead of date. Take a peek at the names on the invitation; if the couple wrote out your name and your plus-one's name, then you can't substitute him/her for another person. On the other hand, if the invitation says "and guest," the couple is encouraging you to bring a casual guest of your choice. Feel free to attend with your bestie, sister, relative, or the guy you've been seeing for a few months now.
Always ask the couple before inviting a mutual friend, though—you never know why they didn't receive an invite in the first place. If possible, use your plus-one's name on the RSVP card so the couple won't be taken by surprise by a stranger at the ceremony.