Net helpmsg 1387 indicates that this exit code means:
A member could not be added to or removed from the local group because the member does not exist.
Usually this is caused by a typo in the credentials that the task runs with.
To view what rights the task ran with open the Agent, go to the Software Delivery or Software Updates tab, open the package that failed, open the Run History tab, and look under the "Run with rights" column. This may show something like <domain>\<domain>\<user> for example if in the policy or task the domain was entered in the "User domain:" field and domain\name was entered in the "User name:" field.
Thanks for your feedback. Let us know if you have additional comments below. (requires login)