Why are some users missing from reminders user selection list?

When trying to set up an event announcement or reminders, there are some users not listed for selection.

When a category is set to option C, viewable only by selected users, then users who do not have permission to view the category will not be listed in the selection list for an event in that category.  There should be a short message at the bottom of the user list which says only users allowed to view the category are listed.

To have a user to show in the reminders selection list, go to Admin Menu, then Categories, then edit the category and click on the user to highlight and give them permission, then save the edited category.  Return to edit the event, the user should then be listed in the reminders selection list.