The structure related to “～者”, “～員”, “～手”
An organization has an objective. It gives its members roles. Each member has his/her own role, different from others. In accordance with the roles, they perform their tasks (activities). When the activities work as designed in a coordinated manner, the objective will have been accomplished.
In this perspective, you can understand some Japanese terminology system of calling people related to the roles/jobs/positions etc, like “～者”, “～員”, “～手”.
事務員(じむいん) office clerk
乗組員(のりくみいん) crew (of a ship/plane/train , etc)
研究員(けんきゅういん) research worker
“～員” means a person who belongs to the organization.
It focuses on the relationship between the member and the organization.
研究者(けんきゅうしゃ) scholar, scientist
医者(いしゃ) doctor (medical)
運転者(うんてんしゃ) driver (of a car)
責任者(せきにんしゃ) person in charge
“～者” means a person who does the activity necessary to accomplish the objective.
This principle can be also applied when there is no organization or objective, such as:
歩行者(ほこうしゃ) walker, pedestrian
信者(しんじゃ) person who believes in something
People do their activity without any organization or objective.
For example, a man can walk by himself, doesn’t need to belong to an organization to do it or has no specific objective. That’s why “歩行者” is correct but “歩行員” is not correct.
When the activity requires some professional skills, the usages “～師” or “～士” are often used.
教師(きょうし) professional teacher
通信士(つうしんし) radio officer
Then, “~者” usage doesn’t mean that the person has the professional skills.
看護師(かんごし) nursing professional (having a qualification)
看護者(かんごしゃ) a person who looks after a patient (qualification is not required)
弁護士(べんごし) professional lawyer
弁護者(べんごしゃ) person who defends the accused
投手(とうしゅ) pitcher (baseball)
捕手(ほしゅ) catcher (baseball)
砲手(ほうしゅ), 射手(しゃしゅ) gunner (military)
騎手(きしゅ) jockey (horse race)
Sometimes “~手(しゅ)” is used for describing a person who belongs to an organization, having a role, expected to do an activity specific to the objective.
This is somewhat alike to “~員”, but “~手” usage is less common than “~員”.