He was on the team during impeachment, specifically for his constitutuional knowledge.
I will present the history of the constitutional impeachment provisions, the history of impeachments that have gone on … and make a broad argument,” Dershowitz told MSNBC’s Ari Melber. “My sole responsibility is to analyze and present the constitutional arguments against impeachment based on the two articles of impeachment
https://www.politico.com/news/2020/01/17/dershowitz-clarifies-role-on-trump-impeachment-team-100740