These are actual cases being held in mock trial. So far, it is my impression in order to keep the cases as close to "live trial" situations as possible, venue is a consideration. For example, I live in Riverside County, Ca. So, I'm thinking I would only receive an invitation to be on an ejury when the actual case is scheduled to be held in the Riverside venue. Again, through the registration process.
Here are links for online jury companies. As I get more info about the companies, I will add it here.
Jury Test EJury Online Verdict Zap Jury Trial Practices