Can I use "a staff" to refer to a person working for an organization or I need to use "a staff member" 用 英語 (美國) 要怎麼說？

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I think you still need to use "staff member" for a countable noun: He is a staff member. We met two staff members.
You could also say "one of the staff" or "two of the staff" in informal English if the context is clear.