The staff they collaborated with were friendly and helpful during the project.

collaborated 是动词 collaborate 的过去式形式。collaborate 由前缀 col-（表示"共同"）和词根 labor（劳动）以及动词后缀 -ate 组成，意思是"共同劳动、合作"。在句中表示过去发生的合作行为。