To share the design with the client:
Select the hamburger button against the desired design's thumbnail.
Select 'Share'.
In the window that opens, under the 'Design sharing (common view access)', a URL link will appear; select the Copy icon against that URL.
The link will be copied to the clipboard, for you to share with the client. (NOTE: Unless they themselves have an Infurnia account, clients can only view and comment on the design, not edit it).
To comment on the shared design (To be instructed to the client/recipient of the link):
Enter the design through the link mentioned above.
Select the Comment icon at the top right to enter the Comment Mode.
Click on the area in the design you want to comment on.
In the lefthand panel that opens, type in the comment and press Enter to apply it.
Select the Comment icon again to exit the Comment Mode.