Gmail, which was launched in April 2004 and is still officially in a testing phase, is not open to the general public.
At first, Google let only select family and friends open accounts at Gmail. But it has slowly opened up membership more broadly to friends of those already holding accounts.
This latest development could indicate that either Google is hoping to significantly expand its user base for further testing or that the company is gearing up for a full launch.
At the time of writing, Google was not available for comment.
Gmail's popularity has already been targeted by online criminals. In September, scammers sent phishing e-mails to existing holders of Gmail accounts, offering them the opportunity to invite three or six of their friends to join Gmail.
The fraudulent e-mail, which claimed to originate from the "Gmail Team," asked people to disclose their Gmail account passwords to receive the additional invites.
Munir Kotadia of ZDNet Australia reported from Sydney.