How to synchronise Drupal users with CiviCRM organisations
I  think it is reasonable to give an organisation a Drupal account. After all, an organisation can also have an email address and a phone number. But I tried to create an organisation-Drupal account combination from the interface on a CiviCRM 4.7.15, but I did not succeed. I tried the following scenario.


I created an organisation with an email address.
I created a Drupal user with the same email address.


Result: two contacts in CiviCRM, an Indiviual and an Organization.

But under the hood, using SQL, you have more options.


Insert or update the connection record in the table civicrm_uf_match.
Update in SQL the type of a contact (the column is contact_type) from Individual to Organization.


I tried this and the Contact View looks fine, and the record is not duplicated when I run the synchronize users job. This suggests that you can do this for a small set with some SQL manipulation, and for a large set you can create an extension.