wmic path win32_group where (LocalAccount=true AND SID="S-1-5-32-544") get Name
edit: that's just the name of the local Administrators group, here's how to get the Admin-accountname:
wmic useraccount where (domain='localhostname' and sid like 'S-1-5-21-%%-500') get sid /value
wmic useraccount where (domain='localhostname' and sid='sidfromfirstcommandhere') get name /value
That will just return the name of the built-in Administrators account. If you wanted to find all user accounts in the administrators group you could do:
powershell -c "$([ADSI]'WinNT://YOUR_MACHINE_NAME/Administrators,group').psbase.Invoke('Members') | foreach { $_.GetType().InvokeMember('ADspath', 'GetProperty', $null, $_, $null)}"