In most industries, women are not only hired at lower rates than men are, they are also promoted at lower rates. A significant portion of this promotion gap remains unexplained even after accounting for observable factors such as productivity. This paper asks whether promotion gaps emerge when employees work in groups and employers cannot perfectly observe employee effort or ability. Using data from academics' CVs, I test whether coauthored and solo-authored publications matter differently for tenure for men and women. While solo-authored papers send a clear signal about one?s ability, coauthored papers are noisy in that they do not provide specific information about each contributor?s skills. I find that men are tenured at roughly the same rate regardless of whether they coauthor or solo-author. Women, however, become less likely to receive tenure the more they coauthor. The result is most pronounced for women coauthoring with only men and is less pronounced among women who coauthor with other women. I test several mechanisms that might explain the result and argue that it cannot be explained by sorting, women taking less credit for their work, or taste-based discrimination.