You can add comments or questions to the document in the same manner as students. Any conversation that you start will be highlighted in blue to distinguish it from student comments (which appear in yellow), so students can easily find your comments if you want to pose your own questions or create annotations to guide students.
You can also respond to student questions and comments and your response will appear alongside student responses. However, you may want to avoid jumping into student conversations, as the presence of an instructor in a conversation can sometimes prevent it from flourishing in the same way it might if students were discussing only amongst themselves.