Share a document library with external users

Thanks Juan Carlos.  I'm not even getting to that stage unfortunately - here's what I'm finding:

1. I'm an owner of the Group and can add guests (external users) to the Group via Outlook.  This gives them access to everything in the Group as expected.
2. If I go to a document library, select a folder or file, click share, select 'Specific people' and type in an external email address (of someone who's not already a member), I get the following error:
   "Your organization's policies on't allow you to share with these users.  Please contact your IT department for help".  Unfortunately I work for a very small company and I AM the IT department!
3. In the Office 365 Admin Center > Settings > Security & Privacy > Sharing:
   1. 'Let users add new guests to the organization' = On.
   2. Under Site Settings, 'Let users share SharePoint Online and OneDrive for Business content with people outside the organization' = On.
   3. Users can share with = 'New and existing external users (sign-in required)'.
4. In the Office 365 Admin Center > Settings > Services & add-ins > Office 365 Groups:
   1.  Let group members outside the organization access group content = On
      [QUESTION: Is it possible to change this setting for one specific group?]
   2. Let group owners add people outside the organization to groups = On

Does this therefore mean that an external guest has to be a member of the Group BEFORE I can share a link to a folder / document with them?  All I really want to do is share the documents within one doc library with specific external users and not anything else.

Really hoping you can help.  Thanks again, Oz