Modify Distribution Group Members (Scripting)

Hi, I am attempting to script the code presented in article  RE: 3669 "Modify Distribution Group Members" to read in a file with DL names and then upgrade them populating the managed by field. I attempted to do this but get an error on the following line, if ran as part of script:
>>>
foreach($g in $groups) { $user = $g.user.tostring().substring($g.user.tostring().indexof("\") + 1);

ERROR: You cannot call a method on a null-valued expression.
At C:\scripts\NEW\DL\NEW\DLmodification-FORLIST-u_test.ps1:32 char:50
+ foreach($g in $groups) { $user = $g.user.tostring <<<< ().substring($g.user.tostring().indexof("\") + 1);
    + CategoryInfo          : InvalidOperation: (tostring:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull


>>>
When I run it manually, it runs fine. Am I missing something? Any Advice would be greatly appreciated.

Attached is my attempted script.

Thanks,
SCRIPT.txt