to start to do something as part of a group, especially something helpful

If we all pitch in together, it shouldn't take too long. 如果我们齐心协力，不会用很长时间。

[ + to infinitive ] When I bought this house, all my friends pitched in to help fix it up. 我买下这所房子时，所有的朋友都来帮忙收拾。

My brother pitched in with an offer of help. 我弟弟主动提出来帮忙。

After we had seen the video everyone started pitching in with comments on its faults. 看完录像后，我们大家都纷纷评论其不足之处。