Mr Varathep, also the deputy agriculture and cooperatives minister, spoke ahead of the first meeting of the assembly at Government House today.
"Conclusions [from the meeting] may not be comprehensive because conflicting parties have not participated," he said.
"However, the [forum] must proceed. Opinions [from other groups] can be heard later."
The Democrat Party, the People's Alliance for Democracy (PAD), and a group of 40 senators said they would not join the reform bid. The eight core PAD leaders resigned on Friday.
Democrat MP Ong-art Klampaibul confirmed on Saturday his party would also not join the forum. He said the forum was a way for the government to distract attention from constitutional amendment and the passage of its amnesty bill.
He said government is insincere in its promise to solve national problems.
Mr Ong-art, however, demanded the forum be broadcast live on television to ensure public participation.
The opposition earlier called on the government to show its sincerity in defusing political conflicts by withdrawing the amnesty bill from parliament.
Mr Varathep said the assembly was still open for the government's opponents to join, although he insisted the amnesty bill would not be withdrawn. It is now being scrutinised by the House.
"However, if the assembly members agree that the amnesty bill should be delayed, they can submit their recommendations to MPs," Mr Varathep said.
More than 50 prominent figures, including former prime ministers, former parliament presidents, political party leaders, academics and representatives of civil society will attend the forum, he said.
Ms Yingluck will today meet Privy Council president Prem Tinsulanonda at his home before presiding over the opening of the political reform forum.
Ms Yingluck is expected to discuss intended political reforms when she meets Gen Prem and will seek his views.
Gen Prem said on Friday he would need time to decide whether to take part in the reform assembly, if he is invited to join it.
Mr Varathep said the government will not set a specific agenda for the forum but would rather listen and gather recommendations on what should be done for the future of the nation.
Topics in the forum will be open, he added. It will not centre on party conflicts but will instead seek to focus on how the nation should progress.
Mr Varathep said the forum may not produce definitive conclusions today and that recommendations could be proposed at a later date. However, today's session should point to the direction of future actions, he said.
He thinks an independent committee can be later formed to work out solutions to the political deadlock.
He said the forum proves the government is sincere in its efforts to work out solutions and he believed results from the forum would be acceptable to the majority of people.
Woothisarn Tanchai, deputy secretary-general of King Prajadhipok's Institute, said the forum would be a way to find solutions and concerned parties should view the forum fairly without mistrust. He will join the forum today.
Senate Speaker Nikhom Wairatpanich said he will also attend the forum. He called on all concerned parties to be open-minded so negotiations for national reconciliation would be successful.
If the government was not sincere about national solutions, it would not have organised the forum, he said.
Noppadon Pattama, legal adviser to former prime minister Thaksin Shinawatra, said his boss would neither share any ideas with the forum nor make any movement for the time being, but would rather listen to ideas from participants and the outcome of the forum today.