Hi Alex thanks for posting. Although it is necessary for someone to have a Lucidchart account before you can add them as a collaborator on a document an alternative would be to publish the document instead. This allows anyone to view and interact with your document even if they don't have a Lucidchart account (although they would need an account to edit or comment on the document.) Please check out our Publish Your Document article for more information about this and feel free to let me know if you have any questions.
Regarding your second question- commenting is a feature that is only available to users on Team and Enterprise level accounts. However a user with a free Lucid account may be able to make other changes to the document if they have edit access. This depends on the content of the document. If someone from a higher level account shares a document that they used paid features on with a free account the free user will automatically be a view-only collaborator since they don’t have access to those paid features. For more information about the features available to free users and other account types please see our Account Types article. I apologize for any trouble this might cause you.