Outlook Connector Showcase (subscribe and send messages)
Manage connectors from OWA or Outlook.
Check full sample code in GitHub.
Guest access to Office 365 groups - Admin Help
https://support.office.com/en-us/article/Guest-access-to-Office-365-groups-Admin-Help-7c713d74-a144-4eab-92e7-d50df526ff96
This is available on Office 365 Small Business Admin
https://support.office.com/en-us/article/Office-365-Groups-Admin-help-3f780e8e-61aa-4287-830d-ff6209cbc192?ui=en-US&rs=en-US&ad=US
From Office 365 Admin Center to enable Outlook Group External Users or Guest to access Outlook Groups.
https://support.office.com/en-us/article/Add-and-remove-group-members-in-Office-365-3b650f4a-5c9b-4f94-a1bb-0cca4b1091de?ui=en-US&rs=en-US&ad=US
https://support.office.com/en-us/article/Guest-access-to-Office-365-groups-Admin-Help-7c713d74-a144-4eab-92e7-d50df526ff96
Guest access in Office 365 groups:
https://support.office.com/en-us/article/Guest-access-in-Office-365-Groups-bfc7a840-868f-4fd6-a390-f347bf51aff6
Office 365 Admin Center
To Manage Groups / Members:
Outlook Groups Guest Access Settings Groups (manage groups/members)
To allow guest access to Outlook Groups:
1) Admin Center Settings services & add-ins Office 365 Groups