admission fee; enrollment fee; joining fee
A fee paid when joining an organization, club, or insurance scheme. Often a one-time payment separate from regular dues.
An admission fee is required to join this club.
The enrollment fee for the insurance is paid only at the start.
入会金 is a near-synonym also meaning 'admission fee', often used interchangeably with 加入金.
Compound of 加入 (かにゅう, 'joining') and 金 (きん, 'money').