title: Calling all SignalR clients in a group from server
body: <p>I need to call all SignalR clients in a group from my server backend. </p>
  
  <p>I need the equivalent of <code>Clients.OthersInGroup('MyGroupName').MyFunction</code> but trough an <code>IHubContext</code> object (or otherwise from outside the Hub instance). Is this somehow possible?</p>
  
  <p>My use case: I've a provider hosted SharePoint App. SignalR is used for real-time updating other clients about changes (using the SignalR JavaScript Client). Since the app supports Multi-Tenancy clients connected to the server can be from different SharePoints tenants. Communication should always only be between clients of the same tenant. Therefor if a client connects it is added to a SignalR group for his SharePoint tenant.
  Now there are certain operations, like showing a document and if it does not exist create it, happening on the server. In this case the client calls an URL of the backend which then returns the document. If it is newly created this should be communicated back to the clients of the affected tenant. </p>