Groothof joined the cast in 1983 as an energetic, always happy young man, with a talent for singing silly songs and telling wonderful stories, and a love for naughty jokes. He portrays the role of a young modern father figure for the Muppets. He also interacts well with children, visiting them in schools or reading them stories at the end of the show.
Besides his work on Sesamstraat, Groothof played various roles in the Dutch children's show Klokhuis (an educational show for a slightly older age group), and starred in the comedic children's show Broertjes (Little Brothers), along with his brother Renee. He is also a stage performer, best known for his operas for children.
From 1987 to 1990, Joekie Broedelet played Frank's grandmother on the show.