There are a few ways to do this:
Using an Alias may be the easiest, but the user would have to be a Domain Admin to create the alias, then edit it when users are added or removed.
A Mailing List may work as well -- if the user is set as the moderator for the list, they have complete control over the subscribers to the list. Just make sure the list is created so that ONLY the moderator can post to the list.
Then there are Categories. When you edit any non-GAL contact, you can simply select a category for them, and a contact can be included in multiple categories as needed.