Hi Karthik,
AFAIK this information is not available in the VBA Office Object Model structure.
And you can not be sure to get this, cause not anybody is logged in:
And it is also possible to use more then one service.
But you can try to access the registry and enumerate all keys in this key:
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Common\Identity\Identities
Have also a look here:
https://technet.microsoft.com/en-us/library/dn151329.aspx
Andreas.