More than 25 MPs from parties other than the 12 that have agreed to merge were on Friday welcomed to the new Jubilee Party by Deputy President William Ruto.
The MPs were from the Raila Odinga-led Orange Democratic Movement, Kalonzo Musyoka's Wiper, Moses Wetang'ula's Ford-Kenya and Gideon Moi's Kanu.
The ODM lawmakers who joined Jubilee include Mpuri Aburi (Tigania East), Isaac Mwaura (nominated), Samuel Arama (Nakuru West), John Waluke (Sirisia) and Steven Kariuki (Mathare).
From Kanu were Prof Helen Sambili (Mogotio), Samwel Moroto, Eric Keter (Belgut) and Kuresoi North's Moses Cheboi.
Those who moved from Mr Kalonzo's Wiper include Regina Ndambuki (Kilome), Joe Mutambu (Mwingi Central), Munuve Mati (Mwingi North), Michael Munyao (Mbooni).
From Ford-Kenya, the team was led by former party national chairman and now Turkana Senator John Munyes.
Other MPs from other parties were Victor Munyaka (Machakos Town) and Peter Shehe (Ganze).
Mr Ruto said more were expected to join the new Jubilee Party.
"We will have more than 41 MPs joining us from other opposition parties tomorrow," said Mr Ruto.
National Assembly Majority Leader Aden Duale introduced the legislators to a rapturous welcome by the crowd at the Kasarani Indoor Arena.
"This is a sign that these leaders have decided that the die is cast that you are either in a national party or you stay in an ethnic party," said Mr Duale.
The DP is expected to make a keynote address at the arena.
President Uhuru Kenyatta will also address the delegates on Saturday morning before unveiling the new party at the 60,000-capacity Kasarani Stadium.