I have a user with permissions to specific host groups, to report to a specific slack channel.

Steps to reproduce:

1. Create host groups intended for private tenancy and segregated permission sets of hosts, lets call this tenant A.
2. Create user with read permissions to tenant A hosts.
3. Give that user a notification media type to post into Slack channel

Result:

The below screenshot shows inaccessible users as the personal who ACK'd the problem.  

It is as though the user with the media associated to the notification, needs elevated privileges to the user listing in order to resolve the name.