It's rude to bring unannounced guests to an event.
Ron is going to a dinner party over the weekend and asks his friend if she would like to join him.
Ron has to decide whether to call the host of the dinner party, or just show up.
Ron calls the host of the party to make sure it's alright to bring a guest with him.
Ron decides that it's no big deal and just brings his friend to the dinner party, with no prior announcement.
The host is fine with having another guest and plans to prepare more food, and Ron's friend enjoys the party and feels welcomed at the event.
The host is concerned that they haven't prepared enough food and Ron's friend feels awkward about being there without an invitation.