Don't have access to Crystal right now, so it is very possible that there's an easyer way to do this, but you could create a formula like

(distinctcount({table.field}, {groupname})/distinctcount({table.field})*100 & '%'

This formula is taking the distinct count per group, deviding it by the total distinct count, multiplying it by 100, and adding the % sign mannually... But again, there might be a simpeler way to do this :)