What is the best way to share a site collection with all users of an organization? Basically make it open to view only. 90% of them would be considered "external users" because they don't have Office 365 or a Microsoft Account associated with their work email address.
If you do not want them to sign using an Office 365 or Microsoft Account, there is no way to provide them access to the entire Site Collection.
It is possible, though, to provide access to individual documents using 'Anonymous Guest Links'. This will of course only be feasible if your site collection contains a limited number of documents.