Google's ( GOOG) email service has stopped playing hard to get.

Google's Gmail, operated on a test basis since last spring, this week dramatically increased the potential number of users of the Web-based service.

The move could mean Google is getting ready for a public rollout of Gmail -- a development that would heighten Google's competition for usage and advertising dollars with Yahoo! ( YHOO) and Microsoft's ( MSFT), both of which offer free Web-based email services as well.

Some subscribers to Gmail -- which since it launched has been available only by invitation from Google or current users of the service -- this week discovered that they had 50 invitations to Gmail that they can pass on to friends and acquaintances.

Previously, Google has doled out no more than a half-dozen invitations at a time to Gmail users.

A Google spokesman confirmed Friday that some users of the service were getting an increased number of invitations, saying it was part of Google's continuing efforts to expand the service. The spokesman, however, wouldn't provide any information about when Google might bring Gmail out of its test phase into general release.

Even in its limited release, Gmail has already remade the landscape of free email service on the Internet. One of the service's distinguishing characteristics is that it provides users with 1 gigabyte of free storage space. At the time Gmail debuted, Yahoo!, for example, was offering users a maximum of six megabytes of free storage.

Since then, Yahoo! has raised its storage capacity on free accounts to 250 megabytes and has upped storage capacity on paid email accounts to 2 gigabytes.

On Friday, Google dropped $6.95 to $203.91.

Senior writer Scott Moritz contributed additional reporting from New York.