After clicking a particular issue (1) that is present in a row, user is to be taken to a Tab view that is dedicated only for that particular issue (2):
It's possible to manage comments by using "Manage" button (3). After clicking it you will see a prompt similar to the presented below:
Please note that a comment is to be added to an issue that is currently marked in the Excel spreadsheet (see above images). When you select multiple issues in Excel and try to add commen tin bulk, there won't by any action after pressing "Send".
After typing a comment and pressing "Send" it is to be added to the issue. Each comment is added in a name of a currently logged in the add-in user.
(TBC)