ProcedureScreenshot
1. Get or prepare the contact list in Google Sheet
2. Copy all the email address from the Google Sheet

1)Select the second row of Email column, Press "CTRL + SHIFT+ Down arrow key") to expand

the selection till the last row of non-blank cell.

2) CTRL + C to copy the values selected

3. At Challenge wizard, select Audience, click on "Add Users by mail" 
4. Paste (CTRL + V) in text box , click on "Subscribe" 
5. You can see the people are invited