2012 Server - How monitor 2008 server on 2012 dashboard

If you want to monitor or create a server group inside Windows 2012 R2 Server about 2008 R2 server you need to install WinRM 3.0 otherwise you wouldn't be able to monitor that server:

http://www.microsoft.com/en-us/download/details.aspx?id=34595

After you download and install the update for the WMF 3.0 run the command prompt as administrator and type “WINRM Quickconfig” to enable remote access: