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。