▲根据英语语法，表示独一无二的职位的名词，其前通常用零冠词。又如：

They elected him leader. 他们推选他为领导。

The party needs a change of leader. 这个党需要更换领袖。

His deputy replaced him as leader. 他的副手接替他当了领导。

he stood down as leader of the party. 他从党首的职位上退了下来。

He emerged as leader at the age of thirty. 他三十岁时初露锋芒担任了领袖。

They are scheming to get her elected as leader. 他们正策划让她当选领导。

She was chosen as leader by common consent. 她是经一致同意而当选为领导人的。

▲你的句子中的 leader 也是表示一种独一无二的职位(这里指主持会议的那个人)，所以没有冠词（即用了零冠词）。

句意：开个小会讨论下面各个主题，大家轮流主持。