register users automatically
You can use feeds module which import data, uses, taxonomy term into Drupal website using CSV file.

In module page:


  Import or aggregate data as nodes, users, taxonomy terms or simple
  database records.
  
  
  One-off imports and periodic aggregation of content
  Import or aggregate RSS/Atom feeds
  Import or aggregate CSV files
  Import or aggregate OPML files
  Create nodes, users, taxonomy terms or simple database records from import
  Extensible to import any other kind of content 
  Granular mapping of input elements to Drupal content elements
  Exportable configurations Batched import for large files