JOIN 动词 有以下意思
加入（组织或机构）；成为…的一员；参加 同义词还有 take part in
If you join an organization,you become a member of it or start work as an employee of it.
they said he would go to college if he (joined) the basketball club
他们说他如果加入了篮球队就可以上大学.
希望可以帮到你!
请留意句子的时态是过去式they said,所以后面的动词也要用过去式.