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Jobs are created at a Client level, and not a Group level, i.e. a job must belong to a Client within the Client Group. This includes all documents and time entries filed for the Job, they must be filed against a Client.
However, depending on your practice settings you may be able to file documents and time against another job that exists for any Client within the Client Group, separate from the current client.
An FYI Admin can remove the green tick for Show Jobs by Client to disable the option. Refer to Managing Practice Settings for Jobs.