While this could depend on your instance's unique configuration, typically when users leave they are set to a disabled status in your Jive instance. This prevents them from accessing the instance, however keeps their content intact and visible to other users.
Does an administrator still have access to the group content they've created and can ownership be reassigned?
There is not an out of the box feature to reassign ownership of content, however depending on the version of Jive you are using it's possible a custom feature could be developed for your instance which would allow for this capability. If you are interested in this option please file a case in your Jive Community secret group and the next available support engineer can assist in reviewed the options available for your specific instance.