See also User Guide.
With SharePoint Online, getting a tenant-wide overview of which site collections are shared with which external users (anyone with a Microsoft account) is hard. Without such overview, and governance in place for managing external users, one risks inadvertently sharing information with too many for too long.
The External Sharing Center (from here on ESC) adds a layer of management and governance on top of the out of the box features. It does so by providing the overview and support for creating and maintaining sharings with metadata such as start and end date. As the end date nears, ESC reminds the inviter that a sharing is about to expire and provides the option to extend it. Once expired, ESC revokes user access from the site collection and sends an access revocation mail to the inviter.
ESC consists of a frontend and a backend part (to keep the design simple, the design doesn't include any custom web service):
A frontend SharePoint web to be installed into any site collection. The web holds pages to display an overview of sharings by site collection and for easy adding and editing of sharing using a guided approach (see end-user documentation).
A background job which monitors site collections and sends out expiration warnings and revocation mails and revokes user access. The job can be setup to continuously import new users and sharings. Otherwise, it threats sharings not already in ESC as unknown and revokes those. Which approach to use depends on whether sharings recorded in ESC or site collections are considered master.
Other notable features of ESC include:
Aggregated expiration warning and access revocation mails grouped by inviter to minimize number of mails sent. Each mail contains a table of sharings and provides a link to ESC for editing.
Overview with security trimmed list of site collections or only site collections with sharing enabled, or a combination of both.
The repository is self-contained and may be compiled by opening the solution in Visual Studio 2015. Compiling triggers NuGet package restore and transpiles the TypeScript which powers the frontend.
The CLI is used to partially setup the frontend, to import existing sharings, to expire user access, and to generate and send mails. The WebJob serves a similar role running in Azure, but only generates mail and expires user access (both backends share a common core).
Before running the CLI (or WebJob), AppSettings inside Bugfree.Spo.ExternalSharingCenter.Cli/App.config must be defined as per below description. Running the CLI in different modes is accomplished by editing Program.cs file:
ExternalSharingCenterUrl. Running CLI in setup mode assumes the hosting web exists and is based on an English (1033) team site template. CLI sets up pages, lists, quick launch, and so forth. In regular mode, CLI processes users and sharings stored in lists on the web.
TenantAdministratorUsername, TenantAdministratorPassword. Iterating site collections and their external users requires tenant-level access using this account and password.
ExpirationWarningDays. When a sharing expires in this number of days, the backend sends a warning mail to the inviter or one in the Owners groups of a site collection in case the inviter cannot be located.
ExpirationWarningMailsMinimumDaysBetween. To avoid repeatedly sending expiration mails whenever the backend runs, this value indicates the minimum number of days between sending warning mails for external users in a single site collection.
MailFrom. Mail is sent using SharePoint's Utility.SendEmail routine, i.e., no SMTP server setup is required. SendEmail requires a site collection context and thus only supports mailing users in a specific site collection. The from address must be a valid mail address as verified by SharePoint Online.
For the WebJob, additional connection strings are required. These are standard WebJob settings and as such unrelated to ESC:
AzureWebJobsDashboard, AzureWebJobsStorage. The WebJob API stores logs and other runtime information inside an Azure Storage Account. Ideally, the account would be named after the WebJob, but the name must be between 3 and 24 lower-case characters and Azure globally unique. bugfreespoextsharingcntr is used inside the config files, but constructing a real name including the tenant is better.
The connection string is available under Storage Account, Keys, and then Primary Connection String. As with any config file connection string, Azure ignores it for security reasons and requires those be setup in the Azure Portal under Application Settings.
On first-time publication of Bugfree.Spo.ExternalSharingCenter.WebJob from inside Visual Studio, a schedule must be setup. Running the WebJob on a recurring schedule at 1am, every night is a sensible default given that user access expires at midnight.
A few manual setup steps are required to finalize ESC setup:
On Start.aspx and Site%20collection%20external%20user%20guide.aspx within SitePages, put the page in edit mode and edit the properties of the Loader web part. Under Appearance, change Chrome Type from Default to None and exit page editing.
Within SiteAssets, delete the existing Notebook file.
Within SiteAssets, create a folder named App and upload the following files from Bugfree.Spo.ExternalSharingCenter.Core/App to it:
Apps.js Loader.js Loader.html SiteCollectionExternalUserGuide.html SiteCollectionExternalUsersOverview.html
Only sharing an entire site collection by authenticated users is supported by ESC. Sharing at a finer level of granularity, such as list or list item, makes gaining an overview even trickier and thus isn't supported. If these finer-grained sharing options become relevant, consider creating a dedicated sharing site collection besides the regular ones.
In principle, SharePoint Online supports external users inviting external users. While ESC doesn't prevent it, when the inviter is an external user, ESC sends no warning and expiration mails to the inviter.
Starting September 2, 2016 emails sent through SharePoint Online originate from firstname.lastname@example.org. Specifying a different from address is ignored by SharePoint Online.
Within ESC, we try hard not to replicate build-in SharePoint dialogs for external sharing. Since we cannot easily hook into SharePoint's invite logic, instead we provide import logic for either one-time or continuous synchronization of SharePoint actual with ESC. In either case, users and sharings are added to ESC with configurable default values, such as some number of days until the sharing expires.
Given the time it takes for the backend to iterate site collections and their sharing, the frontend can't provide up-to-date sharing information except for what it already stores. The ESC frontend therefore doesn't show display name of user, invited as, invited by, and when the sharing is created. Such information would have to be stored inside ESC (in a list or property bag entry) and maintained by the backend.
The frontend's overview page, listing site collections, was originally solely SharePoint search driven. In SharePoint Online, search is the only way for client-side code to list site collections. Unfortunately, search cannot filter on site collections with sharing enabled, causing it to display any site collection which the user has access to. As an alternative, the backend maintains a list of site collection with sharing enabled which the frontend can be switched to display instead. By combining the two options, the frontend can be made to display only site collections to which the user has access and which have sharing enabled.