Drupal 7 Rules - Grant access to node - pick username from node field
Take a look at the Node access user reference module which is meant for this sort of thing (giving access permissions based on a user reference value in the content).


  Gives content access permissions to users for content that references the users with User reference or Entity reference.
  
  This is great if you want your content authors to be able to choose additional authors, or choose who can view their content, as well as being able to display the list of authors or viewers within your content, or in views displays.