Rules/Methods to use for determining user groups with no review data
Start with some natural thresholds : >3* average, >4* good, >4,5*: excellent, 4,9+* perfect. Then you can correct your rating based on some averages, other metrics or even text (but that's hard). Honestly I am not sure it will work as ratings should be viewable and giving different status to customers with same average rating will get noticed. 
Leave them blank, consumers like to know they are first to comment / know there is no reliable rating. No comment is an interesting information. This will push both customers and consumers to give a rating.