Fonseca, 28, will play Frankie, a party girl that Gerald (Josh Peck) meets at a hot nightclub with Jimmy (John Stamos).
When Gerald's one-night stand turns into more, Jimmy worries that his son has fallen in love with a girl who is trouble, reported Entertainment Weekly.
The actress, whose credits also include "How I Met Your Mother" and "Nikita", will appear in at least two episodes slated for November.
"Grandfathered" chronicles the life of a bachelor and restaurant owner who discoverers he has a son and a granddaughter. The series premiered on September 29.