Open a moment and use its reply bar to write a reply. You can also attach photos or a video. Voice replies need Plus, Together or an active Plus trial. If GIFs are available in your app, you can choose one from the attachment picker.
Use Reply on someone’s message to answer that person inside the conversation. Type @ and choose a person to mention them. Notifications follow each person’s settings.
Shared notes and guestbooks also have a reply bar. A reply in a guestbook that is still being collected stays hidden from the person the book is for until it is given.