Ackman's hedge fund Pershing Square Capital Management owns 8.3 percent of the company, making it a top 10 shareholder, along with mutual fund giants Vanguard Group, BlackRock Institutional Trust Company and State Street Global Advisors.
Two days after meeting the full board for the first time, Ackman on Thursday tried to cool the emotionally charged atmosphere. His hedge fund sought three board seats and has proposed expanding the board from 10 members to 13. The company has rejected the proposal.
Ackman has been pushing ADP to streamline operations. He made a 3-1/2 hour public presentation last month outlining the kind of operational improvements he thinks will help boost the share price.
In a letter to the board, he wrote that a "substantial number of shareholders ... would strongly prefer that the company embrace its significant potential for improvement and that we work together in the best interest of the company."
The letter was made public in a regulatory filing.
Ackman, one of the world's most prominent activist investors, got off to a rocky start in his relationship with ADP, a human resources software company worth roughly $50 billion.
ADP said Pershing Square wanted to control the company and replace Chief Executive Officer Carlos Rodriguez, who went on television and called Ackman a "spoiled brat."
Ackman has tried to calm the waters. But his comments that shareholders want the sides to work together might be a signal that other large investors may not support him in an expensive, time-consuming proxy contest.
Ackman has tried for some time to settle the fight before the Nov. 8 annual meeting. He told the ADP board as well as Wall Street analysts that he wants to avoid taking this fight to a vote.
Even if there is no settlement and shareholders vote, Ackman has told people he expects to win at least one seat. He cited Pershing Square's stake and its strong relationship with proxy voting firm ISS which makes recommendations to shareholders on how to vote, a person who spoke with him said.
The company's share price climbed 1.3 percent to $109.34 on Thursday.
(Reporting by Svea Herbst-Bayliss; Editing by Bernadette Baum and David Gregorio)
