Individual ownership is clearly control over a resource, including the right to exclude others.
"Joint ownership" in a libertarian society requires a contract detailing the rights of the joint owners. In some cases this will be a democratic model, where majorities are required to trade the resources, allow new people to use it or to join the group, etc.; in others, there will be more defined roles (e.g., a corporation with a board of directors and so forth, which have more rights to control the resource than other shareholders, with those shareholders having a right to change the board by majority vote by shares represented).