How Do I Decide Who Can Bring a Date?
You should extend a plus one to anyone who is in a committed relationship, whether married, engaged, or in a live-in partnership—even if you haven’t met the other half. You are not obligated to give single guests and guests who are involved in more casual relationships the option to bring a date. You do, however, want to be consistent and avoid making exceptions.
If the invitation does not say, for example, Anna and guest, guests can assume they cannot bring a date. If someone does show up with an uninvited guest, avoid an uncomfortable situation by finding a place for them and follow up with the invited guest via a polite phone call afterward.