Ashton Kutcher and Mila Kunis reunited with their That '70S Show castmates Topher Grace, Wilmer Valderrama, Laura Prepon and Danny Masterson on Tuesday (08Oct13) to catch up with their old pals. Masterson shared a photo of the actors singing the series' theme song, alongside the caption, "Every once in a while friends gather for a 6 part harmony of a Big Star song..."