application to participate; registration
Used for signing up for events, courses, conferences, etc. Often appears in formal contexts.
Registration for the seminar has closed.
Please fill in the required information on the participation application form.
申し込み is a general term for application or registration, while 参加申し込み specifically means application to participate in something.
参加 means participation itself, whereas 参加申し込み refers to the act of applying to participate.
Compound of 参加 (participation) and 申し込み (application).