Sonia Gandhi congratulated all six MLAs on the occasion.
Congress' Rajasthan in-charge, Avinash Pande said: "These MLAs have been with the party, but now with their joining the party, the state government in Rajasthan has become stronger."
Soon after joining the party formally, MLA Ranveer Singh Guda said: "I shall stand strong to help Congress in difficult times like 'Hanuman'. Even in 2008, six BSP MLAs had joined the Congress," he added.
The six MLAs include Guda from Udaipurwati, Wajib Ali from Bharatpur, Joginder Awaana from Nadbai, Sandeep Yadav from Alwar, Deepchand Kheriya from Kishangarhbas and Lakhan Singh from Karauli.
They were given the Congress scarf by Pande.
The sudden shift of the BSP MLAs towards the Congress in September had jolted BSP chief Mayawati.
While Sonia Gandhi had given her in-principle approval for the induction of these MLAs into the party, state Congress chief Sachin Pilot was reportedly not kept in the loop during the entire episode, confirmed party members.
In September, the six BSP MLAs met Rajasthan Assembly Speaker C.P. Joshi and handed over a letter informing him of their decision.
Guda at that time had said that all MLAs had joined the congress to fight "communal forces" and to ensure stability of the state government.
Soon after 2018 Assembly election, the BSP had announced to extend support to the Congress from the outside. However, Awana said that it was difficult to give external support to the party.
State BJP spokesperson Amit Goyal said the Congress has a history of "buying" MLAs. Gehlot had cheated Mayawati around 10 years ago also, and he did the same now by buying all six MLAs, he said and added, "It's sad the BSP didn't learn from the past."
"Gehlot was known for his political greed and not following any norms," Goyal said.
While Gehlot showed his guile by announcing the merger without letting state Congress chief Pilot getting a whiff about it, now Pilot is responding by delaying their membership, the BJP leader said.
"This speaks volumes about the Gehlot vs Pilot politics and lack of coordination between the ruling party and the government," Goyal said.
( With inputs from IANS )