General term for someone new to a field, activity, or organization.
I bought a guidebook for beginners.
new pupil; new disciple
Specifically refers to a new student or follower in a school, dojo, or similar setting.
A new disciple joined the dojo.
This course is ideal for newcomers.
新人 refers to a newcomer in a workplace or team, often implying a probationary period. 入門者 is broader and can refer to any beginner in a discipline.
Compound of 入門 (にゅうもん, 'entering a gate' i.e., beginning study) and 者 (しゃ, 'person').