admission (to a society, club, etc.); joining; entrance; enrollment; enrolment; becoming a member
「入会」 has another reading with different meanings.
彼は新しいテニス部に入会したばかりだ。
He has just joined the new tennis club.
そのジムは入会金が非常に高かったので、入会をためらった。
The gym's enrollment fee was very high, so I hesitated to join.
彼女は来週、ボランティア団体への入会手続きを済ませるだろう。
She will probably complete the enrollment procedures for the volunteer organization next week.