Writing is not only a tool for communication but can assist in generating, elaborating and structuring knowledge, which is commonly referred to as writing-to-learn. This can be beneficial in many mathematical learning processes. However, most research on mathematical writing has focused on individual writing-to-learn in problem solving activities. In contrast, only few studies considered possible benefits for the fostering of mathematical argumentation and proving, particularly when writing-to-learn occurs in collaboration. We review fundamental principles of writing-to-learn in mathematics education and elaborate how it could be implemented in collaborative learning settings. We refer to previous research on fostering mathematical argumentation skills as well as to theories of writing-to-learn from other areas within mathematics education. Finally, we provide guidelines for designing collaborative knowledge-constituting writing activities, discussing the importance of prompts and guidance.