Mr Niwattumrong arrived at 9.35am, followed by Chalerm Yubamrung, former labour minister and director of the dissolved Centre for the Administration of Peace and Order, and Tongthong Chandransu, permanent secretary of the PM's Office. Soldiers stayed with Mr Chalerm since Thursday night.
Those who reported in at the army auditorium earlier were former deputy commerce minister Yanyong Phuangrach and former energy minister Pongsak Raktapongpaisal.
Mrs Anchalee Paireerak, a leader of the People's Democratic Reform Committee, also reported in at the auditorium on Friday morning - the first leader of the anti-Yingluck Shinawatra government movement to show up.
Mr Niwattumrong and Mr Chalerm were among 23 people the NPOMC summoned on Thursday. Others include former prime minister Yingluck, her elder sister Yaowapa Wongsawat, former prime minister Somchai Wongsawat, Noppadon Pattama, Pokin Polakul, Pichai Naripthaphan, Phumtham Vejjayajai, Gen Chaisit Shinawatra, Phansak Winyarat, Suranand Vejjajiva, Pol Gen Worapong Chewprecha and Pol Maj Gen Thawat Boonfuang. They were all connected with the toppled Yingluck government.
Early Friday morning soldiers of the 1st Division, King's Guard, prepared the auditorium to receive the summonsed people and kept reporters outside at Yaek Phol 1 intersection, adjacent to Nakhon Ratchasima Road, which is also generally off limits to public and private vehicles.
A rapid deployment force of the 31st Infantry Regiment, King's Guard, occupied the army auditorium on Friday morning.