On the Add Guests page, choose 'Import Contacts’ and then select the ‘Upload a file’ option.
If you are having trouble: The most common issue when uploading is that the CSV file is missing the column titles. The file will need to have at least one column with the title "E-Mail Address". Open your file in Excel and make sure to add this column title if it's not already there. Other columns titles "First Name" and "Last Name" will also be imported if present. These column titles must be in the cells at the very top of the spreadsheet.
After all of the column titles are in place, try uploading again. Make sure to attach the file that is giving you trouble.
To create a .CSV file from Excel: First open the spreadsheet in Excel. From the "File" menu, choose "Save As", and then change the "Save as type" to "CSV (Comma delimited) (*.CSV)". Click Save. Now you are ready to upload the CSV file.