Commenting
Besides real-time collaboration, Balsamiq offers many other ways to get feedback on your designs. To learn more about it, check out Sharing and Reviewing.
Among those, the Comments Panel is a useful option that doesn't require edit permissions.
A red dot indicates that a Comment is unread. Comments can be marked as read and unread by clicking the blue/grey dot. You can add Callout markers to indicate specific elements in the wireframe where the comments applies or add a link to an alternate version.
You can learn more about comments in Reviewing and Commenting.