Working with SID

technet more info

Account Name to SID

#
#Get "fabrikam, kenmyer" to SID
$objUser = New-Object System.Security.Principal.NTAccount("fabrikam", "kenmyer")
$strSID = $objUser.Translate([System.Security.Principal.SecurityIdentifier])
$strSID.Value

SID to Account Name

#
# translate "S-1-5-21-1454471165-1004335555-1606985555-5555" to Account Name
$objSID = New-Object System.Security.Principal.SecurityIdentifier `
    ("S-1-5-21-1454471165-1004335555-1606985555-5555")
$objUser = $objSID.Translate( [System.Security.Principal.NTAccount])
$objUser.Value