Any viewer can add comments or post to existing comments on diagrams, resources, data structures, requirements, stories and reference models. Any comment thread can be marked "Resolved".
For diagrams it is a top right icon:
In most objects the COMMENTS are accessible in the right panel. (On more sophisticated objects you may need to scroll the menu items to the right using the > to get to "COMMENTS".)
Seeing new comments on your home page
You can see items which have had comments added (or have been edited) by following them. All followed and favorite items are displayed in the main body of the home page. They are initially ordered by the date you started following them. If any of them are edited they will move to the top of the list.
If you switch the toggle "Show favorites only", the home page will be cleared to show only your favorite followed items.
If there is any new comment on the followed item you will see a blue comment icon. If the item has been edited by another user since the last time you saw it, it will display a blue pencil icon.