noun
Refers to someone who has recently joined a group, organization, or community. Often carries a nuance of being inexperienced or unfamiliar with the ways of the group.
彼はまだ新参者だから、仕事のやり方を覚えているところだ。
He's still a newcomer, so he's learning how things are done.
新参者の私に、先輩が丁寧に説明してくれた。
My senior colleague kindly explained things to me, a newbie.
初心者 means 'beginner' and focuses on lack of skill or experience in an activity. 新参者 is about being new to a social group or organization, not necessarily a beginner in the field.
Compound of 新参 (しんざん, 'newly joining') + 者 (もの, 'person'). The exact historical derivation is uncertain, but the word has been used since at least the Edo period to refer to newcomers in a group.