no plus ones

Often seen on a party invite (usually followed by -"there will be a list"), giving instruction that no uninvited people are allowed to attend apart from that one person everyone knows but doesn't really know to come to a party.
he'll be at every party and it will inevitable that he will come.

hey guys come to my party!
no plus ones, there will be a list

but then this fearless youth will arrive uninvited and he will not be asked to leave