In my team only a few team members attend Product Backlog Refinement while others work on current Sprint items, is it fine?
It is up to the Scrum Team to determine how PBR will be performed. The readiness of the PBIs being selected for the Sprint would be a good inspect and adapt point to consider the efficacy of a particular approach to PBR.
In my experience, having all team members attend PBR will help reduce complexity as good brainstorming happens and everyone will have same level of understanding on the PBIs from the discussion.
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