How to politely say in a paper that most of the published research on a topic is by the same person?
First off the fact that one group is dominating will be implicitly understood if you provide a paragraph or two summarizing their work and cite their publications. In such a paragraph you could add words like "ground-breaking" or phrases that describe the dominance of the group on the field. 

You could for example start the paragraph by saying: 


  "The work concerning risk assessment of [...] has largely been carried out by [the research group] (example citations)." 


(I hope you see the main idea of my attempt) Then you follow up on what they have done and cite their work.

It is just important to keep the tone as neutral as possible and let the citeable work speak for itself. Spending one or more paragraphs describing the work should be enough to make everyone understand the message.