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If you have shared the document then the context menu on the comment box shows an Email collaborators… option.

As does the Share button

The option sends an email to everyone who shares editing rights for the document -  note that the option won’t appear until it has actually been shared. The email will contain a link to open the shared doc. You can also include a message as well.

Once the document has been shared - everyone who has opened it is added to the collaborators list, so this includes users who might access documents through a shared folder on a cloud drive for example. It also means that you don't have to keep on laboriously adding email addresses to the share list. 

You can also see who's on the list in the Current collaborators field.

Have a look at these other articles for more info on document sharing and collaboration...

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