Hi, my suggestion is that 被 cannot be used in every situations that has passive meaning. In Chinese, 被 is usually used to describe negative results, or results that you don't desire. For example, 小王的自行车被偷了。他被他爸爸打了一顿。我的咖啡被他喝光了。
事-解决；书-还给张经理；会议-举行 are quite neutral, not negative at all. So we'd avoid using 被 here.
Alternatively, you may say:
李先生 主持(host) 了今天的会议。 举行 is for large events, like annual meeting, sport games, election, etc.