Groups have extra functionality not seen on private memories and this is the ability to add a voice memo to someone else's memory, and here is how to do it! In the news feed section of the group, click the View Full Memory button underneath the memory. Click on the Voice Memo option on the right-hand side: Start recording your voice memo and don't forget to save the recording when complete by pressing the red timer button. This will let others hear what you have said about the memory. See the How do I listen to voice memos on a memory? to see how to listen to other memos.