Grooming

Refinement usually consumes no more than 10% of the capacity of the Development Team.

However you measure capacity, 10% of is usually allocated to refinement activities. Personally, I tend to use working hours as a rough measure - 4 hours times the number of members of the Development Team per week of the Sprint. If I was being more precise, the Development Team doesn't have a capacity of 40 hours per week every week in the Sprint - some time is spent in Daily Scrum, Sprint Planning, Sprint Review, and Sprint Retrospective. It's probably closer to 3ish hours per week per Development Team member per Sprint.

Regardless, unlike other Scrum events, refinement isn't timeboxed. It's a rough approximation. Sometimes, a team can refine a sufficient portion of the backlog without using the full 10% of the capacity. Sometimes, the team will need significantly more time. Sometimes, most or all of the team will be involved in refinement. Othertimes, a subset of the team may do the refinement activities. I use the 4 hours per person per week of the Sprint as a high level baseline and spending significantly less or significantly more for an extended period of time would raise concerns that should be discussed with or among the team. Concerns from the Product Owner on the number of Product Backlog Items ready for refinement may also raise concerns.

I have a question, does refinement participate 10% of the development team or is it 10% of the team time?

If 10% of the Development Team were designated to be Product Backlog refiners, how well do you think that arrangement would work out? What issues would be likely to arise?