Visit the "Recruit your own testers" tab of a test, and then turn on the request demographics feature located just below the share test link. Once the feature is turned on you can select which demographic data you want to collect.
Demographic surveying only apply to testers recruited by you, and not those ordered from UsabilityHub.
Before the test begins, your recruits are given a simple survey to complete asking for the demographics you requested. All fields are optional.
Demographic surveys are available on the Pro and Team plans.