Computational notebooks aim to support collaborative data analysis by combining code, visualizations, and text in a single easily shared document. Yet, as notebooks evolve and grow they often become difficult to navigate or understand, discouraging sharing and reuse. We present the design and evaluation of a Jupyter Notebook extension providing facilities for annotated cell folding. Through a lab study and multi-week deployment we find cell folding aids notebook navigation and comprehension, not only by the original author, but also by collaborators viewing the notebook in a meeting or revising it on their own. However, in some cases cell folding encouraged collaborators to overlook folded sections or spend longer reviewing a notebook before editing it. These findings extend our understanding of code folding's trade-offs to a new medium and demonstrate its benefits for everyday collaboration. We conclude by discussing how dynamic reorganization can support sharing and reuse of computational notebooks.