Wedding companion

PLUSONE

This person might be invited when your date isn’t already on the guest list—often seen on RSVPs with a dash.

Wedding companion" hints at someone invited to a wedding as an extra guest, often when the main invitee doesn’t know many people. The term "plus one" is commonly used on invitations to mean a guest can bring another person, and here it’s written as a single word, "PLUSONE," to fit the crossword format. The clue plays on the literal meaning of adding one more to the guest list.