Click on your avatar at the top right of the page, then click Site Admin.
Click Invite Users on the left.
In the Invite users via Email box, enter one email address per line. If you have a CSV with a list of email addresses you would like to use, click Import CSV and choose the file.
Customise the message that will be sent.
Click Invite Users. We'll sent an email with your message to the addresses you provided.