你看看韦氏高阶动词join的释义之一和例句：（注意我加粗的二个例句）
: to become involved in or part of (something, such as an activity)
[+ object]
I joined the line and waited patiently to buy a ticket.
Hundreds of people have joined the effort to save the building from demolition.
He agreed to join the debate.
[no object]
— + in
Hundreds of people have joined in the effort to save the building.
She joined in the conversation/fun.