I want to be able to have individual users send messages to each other using SignalR, therefore I need to send to a Specific Client ID. How can I define the client ID for a spec
Replace the IConnectionIdFactory with your own https://github.com/SignalR/SignalR/wiki/Extensibility.
Sample usage: http://www.kevgriffin.com/maintaining-signalr-connectionids-across-page-instances/
EDIT: This is no longer supported in the latest versions of SignalR. But you can define a user id for a specific connection using the new IUserIdProvider