noun
registrant; subscriber; enrollee
Refers to a person who has registered for a service, event, course, account, mailing list, channel, or similar system. In online contexts it often means a subscriber or registered user.
このサービスの登録者は百万人を超えた。
The number of registered users for this service has exceeded one million.
セミナーの登録者に案
We sent an information email to the seminar registrants.
彼女のチャンネルは登録者が増えている。
Her channel is gaining subscribers.
加入者 emphasizes someone who has joined a plan, service, organization, or insurance scheme; 登録者 is broader and can simply mean someone entered in a registry or system.
会員 means a member of an organization, club, or service; 登録者 may only be registered and does not always imply membership privileges.
A Sino-Japanese compound formed from 登録, meaning registration or enrollment, plus 者, meaning a person; literally, a person who is registered.