Any advantage of group CBT over one on one CBT therapy?

Generally. both are used concurrently! Seek Professional Psychology Department eg Teaching Hospitals for a program tailored for your needs!! My Opinion only Good Luck Dr Z.
Less evidence. As there are less studies behind group vs individual CBT, but for some individuals a group setting may be more beneficial(and more comfortable) than an individual setting. Who that is depends on a number factors including the primary diagnosis and need for CBT and any complicating diagnoses. This is best left up to your therapist/psychiatrist obviously with your input to help guide decisions.
No advantage. Group CBT is efficacious but outcomes may be superior with individual treatment. The choice depends upon patient preference and the availability of resources, and health psychologist.