Most of the guests ----- to Betty's birthday party were her

这里是一个定语从句,定语从句又是对Most of the guests在起限定作用,意思是：大部分被邀请的客人,邀请的主语是Betty,宾语是guests,guests是动作的承受者,是被邀请的,所以原句应该是这样的,Most of the guests _(who were )invited__ to Betty's birthday party were her classmates.定语从句的主语谓语可以省略,所以省略了就是原文的Most of the guests _invited__ to Betty's birthday party were her classmates.