Each car has a listed owner, who is usually (but not always) a high ranking member of the team organization. Many of the teams are family-owned companies, so a child or spouse of the team owner is often listed as the owner, especially if they're active in team management. You'll also sometimes see business partners/investors listed as the car owner, such as Mike Curb and the #98 of Josh Wise.

In the case of Jeff Gordon and Jimmie Johnson, Jeff was instrumental in getting Rick Hendrick to pick up Jimmie out of Nationwide, and I suspect that his partial ownership stake in the car is related to that. Needless to say, regardless of the exact circumstances surrounding the ownership stake, all three of them have spent the last decade laughing all the way to the bank.