When trying to set up an event invitation, announcement or reminder, there are some users not listed  in the recipients list.
 

• If a category is set to "C. Selected Private: viewable only by selected users," then users and subscribers who do not have permission to view that category will not be listed in email recipient list.
• If a category is set to "B. Private: viewable to logged in users only," then subscribers will not appear in the recipients list by default.
   

  *  By default subscribers may receive event emails for public events only. Administrators can override this with a global setting via: Admin Menu > options > advanced tab > check "Allow private event emails to any user or subscriber" > SAVE

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.