Comment on mems
To comment on a mem, highlight the text you want to comment on, hover over the lightning bolt button to the right, and click "Comment" from the dropdown. From here, you can choose to keep the comment private, or post for everyone to see. Each comment is also its own mem, so that every comment you make becomes a part of your searchable knowledge.
Keep a comment private if it's for your eyes only.
Post as a comment to share it with the collaborators on the original mem — and let them reply in that comment thread.