You can upload attendees information through a CSV file. You can full details on how to do that here.
If you're having issues with your CSV file, there are a few troubleshooting steps that might be helpful for you.
Check the columns headers:
The headers in the CSV files are case-sensitive, as they are linked to similar columns in the database. In your file, make sure that the headers match the original naming in the template (i.e. firstName NOT FirstName or Firstname).
Make sure that all default fields are populated:
Each attendee must have an email added in the email column, while it's not necessary to have both first name and last name for every attendee, it is a lot better to process if both columns were populated.
If you don't know their first or last name using an initial or adding a dash "-" is a good substitute.
Check that the columns are in the right order:
The columns in your CSV files should be in the following order (email, firstName, lastName, code, etc.)
Make sure that your custom-data is unique:
If you're using custom data, make sure that the headers of your custom data are precise and unique.
Avoid using any duplicates (i.e. having two columns called department), or system-keywords (i.e. using reserved words like begin, end, etc. as header names).
Make sure that your file in encoded correctly:
When dealing with CSV files, your computer may encode the file differently. So it's important to make sure you use UTF-8 when you save your file.