Google has extended its Google Glass trial in the US, with an undisclosed number of existing beta testers being invited to get a friend involved in the trial.
The company emailed invites to current "Glass Explorers" to encourage one friend to become a Google Glass owner too, providing they pay $1,500 (£985) for a pair of the augmented reality glasses.
There are an estimated 10,000 owners of Google Glass; around 2,000 ordered a pair at the 2012 Google I/O conference, and a number of people won them in this year's #ifihadglass contest.
The emails, which Google says are no longer being sent out, allowed current owners to invite one friend to join them in the trial. The person had to be at least 18 years old, living in the US, and able to pick up their Google Glass device in either San Francisco, Los Angeles or New York.
The search giant originally suggested the product would cost up to $600 (£396), but the considerably higher pricing of the Explorer Edition already being worn by 10,000 trialists has drawn that figure into some doubt.
An alternative to Google Glass recently appeared in the shape of the GlassUp eyeglasses, but they don't include native voice control or a built-in camera, and can only be used with a smartphone.