NPP spokesperson James K Sangma announced at the rally that along with them, 10 members of the tribal autonomous district councils, have also formally joined the party.
The NPP is headed by Conrad K Sangma, son of former Lok Sabha Speaker P A Sangma.
The eight MLAs, including five of the ruling Congress, had resigned from the state Assembly on December 29 last year and announced that they would be joining the NPP, a constituent of the BJP-led North-East Democratic Alliance (NEDA), a political coalition that was formed in 2016.
The MLAs, who joined the NPP, are former deputy chief minister Rowell Lyngdoh, Sniawbhalang Dhar, Comingone Ymbon, Prestone Tynsong, and Ngaitlang Dhar of the Congress, Remington Pyngrope of the United Democratic Party and Independents Stephanson Mukhim and Hopeful Bamon.
In his address at the rally, NPP president Conrad K Sangma expressed "happiness" at the formal joining of the eight MLAs and said that the Assembly 2018 is about "fulfilling the dreams of my (Late) father Purno A Sangma and it will be tough".
"2018 will be tough but I am sure you all will work hard and come out successful," he said.
NPP spokesperson James K Sangma said the NPP stands for the "change" that people are looking for.
"The NPP is the party that will bring in the change. A party that will fight for the minorities in the country - the Christians, Muslims, Sikhs, Seng Khasi and the Songarek (tribals organisations in Meghalaya)," he said.
Former Meghalaya deputy chief minister Rowell Lyngdoh, a veteran Congressman who was elected six out of the nine terms of the Assembly in the state, said, "Meghalaya expects a change and we have to vote out the present Congress government under Chief Minister Mukul Sangma."
Manipur deputy chief minister and NPP leader Yumnam Joykumar, who was present at the rally, highlighted that the NPP is a party originated from Manipur, rubbishing claims that the NPP is a Garo party.
He said the NPP as envisioned by (late) P A Sangma stands for the North East people and the people of the region as a whole.
Today's development came close on the heels of four MLAs, including former Congress legislator, Alexander L Hek, joining the BJP two days back.
Apart from Hek, former deputy speaker Sanbor Shullai (who also headed the NCP in the state until last year) and two Independents -- Justine Dkhar and Robinus Syngkon -- joined the BJP at a rally at Shillong on January 2.
Joining of MLAs have boosted the BJP and its allies ahead of the Assembly elections in Meghalaya to be held soon.
Meghalaya at present has the Congress government with Mukul Sangma as the chief minister.
Term of the 60-member Meghalaya House is ending on March 6 and poll process has to be conducted before that.