这是动词 attend 的过去分词形式或被动语态形式,可以是谓语的一部分,也可以做句子的伴随状语或后置定语.
谓语的一部分：The meeting was attended by lots of students this morning.
伴随状语：There was a very popular meeting this morning,attended by lots of students.
定语：I enjoyed this morning's meeting attended by lots of students.
这种形式不被看作是“短语动词”或“动词短语”.