If the organisation was uploaded with its employees through a spreadsheet, they will all be placed under the same organisation as long as the name is spelt exactly the same, with the same letter casing. 

For example, if you wanted to place 3 attendees under an organisation called 'Mann, Co.', you must keep all the same casing and punctuation. If the organisation name was spelt as 'manN Co.' or 'Man, Co' for some attendees, they will be placed under entirely different organisations. 

For more information on importing data using a spreadsheet, have a look at this article.