Hi Scott

Thanks for contributing to the Lucid Community.  Unfortunately there is not currently a way to tag all document collaborators at once when commenting. However we're always looking for ways to improve Lucidchart and your feedback is the best way for us to do that. Would you mind adding your idea to our feature request form? Ideas submitted to this form are passed on to the product development team for their consideration.

Although this doesn't concern commenting it is important to note that you can share a document with an entire group of users so long as they're all assigned to the same group by your Lucid admin. To do this simply list the name of the group in the Share menu (e.g. for a group called "North American office" simply enter the group title in the Share menu to add all members of that group as a collaborator to the document). For more information about group membership please review this article from the Lucid Help Center (only manageable by users with Team Admin Group Admin or Account Owner privleges). 

Thanks for your request and I apologize for any inconvenience this causes you.