1.表示【加入/参加(组织或团体)】,是及物动词,直接用【join sth】,英语解释为If you join an organization, you become a member of it or start work as an employee of it. 例如:
join the league/army /party入团/参军/入党
2.表示【参加某种活动(多指正在进行的活动)】,英语解释为If you join an activity that other people are doing, you take part in it or become involved with it. 参加 (活动)。
3.搭配有两种:join sb (in sth)【If one person joins another, they move or go to the same place, for example, so that both of them can do something together. 和…一道】;join sb in doing sth
例如:We'll go to the beach for a sunbath,will you join us?;Will you join us in the game?;Will you join us in play basketball?可见,当表示【加入/参加(组织或团体)】时,join后面没有必要用介词in.原题中,语境是:It's not necessary to 【join a health center 】to keep fit.为了保持健康,没有必要【加入健康中心】。这正符合join的第一种用法,所以,不需要用in。