The Slingbox and Slingplayer only allows one person and one video source to be viewed at any given time. This is actually a feature called SlingSecure described here: (www.slingmedia.com/get/io_1255477160645.html)
SlingSecure means placeshifted content is secured and limited to only one authorized user at a time
There is an application called, mReplay Broadcast, which allows you to restream a portion of your desktop as a virtual web camera. This would all you rebroadcast the Slingplayer window over Skype, MSN Messenger, Justin.tv, Ustream.TV, or any application which allows web camera streaming.