。
She applied to join the golf club.
Literal
She [topic-は] golf-club [to-に] membership [object-を] applied.
申し込む ('apply for, propose, register') is a productive verb covering everything from job applications to marriage proposals to subscription requests. The に marks the target of the application — the golf club she's applying to. 入会 ('entering an association, becoming a member') is built from 入 ('enter') + 会 ('association, gathering') and routinely pairs with 申し込む or する. Japanese golf clubs traditionally require formal membership processes, and applying often involves vetting, sponsorship, and significant fees — much more involved than simply showing up to play. The 会員制ゴルフクラブ ('member-only golf club') is part of midcentury Japanese business and leisure culture.