self-seaters
A group of people who seat themselves at a table in a restaurant where a host or hostess is meant to seat you, thereby causing an inconvenience to the wait staff. Can also be "self-seater" singular for one person.
"Oh, there's some self-seaters on the patio, how annoying".
"Who sat those people there?" "Nobody, they are self-seaters "
"Who sat those people there?" "Nobody, they are self-seaters "