It is very possible that the person you wish to add as a subscriber doesn't currently have access to the event category you are adding your document or discussion to.
You can search for a specific user by entering their name into the available search box to check if the user you are looking for has access to the particular event category you are adding your conversation to. Any available users matching your search criteria will be displayed in this area. If you are still unsure, please contact your event Administrator.
Admin users - if you can't find a specific user when adding subscribers, you can always invite them to join the conversation by selecting the 'Invite' option found on the page. This is displayed on the left column of the page and when no results are found when searching for a subscriber. Simply click on this option, enter the user's email address, and complete the necessary actions to add them as a subscriber to your new conversation.
Note: Users invited via the subscriptions manager will only have access to the current conversation area and event you are interacting with. If you wish to manage their event and area access, you must enter the 'Users' area of the Event Dashboard or the 'Manage Users' area of the Admin Property Dashboard.