noun
subscriber; member; participant; affiliate
Refers to a person or entity that has joined a service, organization, or scheme. Common in contexts like telephone/internet subscriptions, insurance policies, and membership programs. The specific English gloss depends on the domain.
新しい電話サービスの加入者が増えている。
The number of subscribers to the new phone service is increasing.
保険の加入者は
Policyholders should contact us immediately if an accident occurs.
この組合の加入者は、毎月会費を払う必要がある。
Members of this association must pay monthly dues.
契約者 is a contracting party, focusing on the legal agreement, whereas 加入者 is the person who has enrolled or subscribed, often used in insurance and telecommunications.
Compound of 加入 (かにゅう, 'joining, subscription') + 者 (しゃ, 'person'). The meaning is transparent from its components.