There are companies that run focus groups. I've been selected to attend a few of them, as has my spouse. Usually an ad is placed somewhere and people are vetted in a phone interview or online survey. The vetting process chooses people in the target demographic for the group. (such as "people caring for a family member with cancer" or "women who have remodeled their kitchen in the past 3 years" or "men between the ages of 30-45 who are thinking about purchasing a van in the next 12-18 months"). Sometimes the groups are rather broad, sometimes extremely specific. They are usually paid for their time, and the payment is quite generous.
The group is brought together and shown various things and is asked questions about them. Products, options and features, advertising, etc.
A focus group is generally geared toward product/feature development or marketing and advertising (I once sat in on a focus group for Celebrex ads, for example).
That is different from a user test, where people fitting the different user personas are brought in to try different interfaces or tasks. User testing is ideally done in a lab of some sort, with sophisticated eye tracking and/or video recording (and screen capture) equipment. Of course, user testing can also take place under much more modest circumstances, and often does since many companies don't want to spend the resources to conduct expensive and detailed user testing.