Lamba tweeted: "Time to Say Good Bye to Cong n its Leadership after spending 20yrs with them, Lastly I would say Thanks to Cong President Smt.Sonia Gandhi."
The NSUI is the Congress party's student wing.
Lamba has reportedly met AAP leader Yogendra Yadav over the issue of joining the party.
"Mr Yadav confirmed that he had met Ms Lamba along with members of several other parties," NDTV reported.
"While all are welcome to join it, a decision on how best to utilize them is decided after a primary member has been engaged with the party for a few months," the news channel reported quoting AAP sources.
Fifteen days after the Delhi assembly election results were declared, the AAP this month said it will form the government with the support of the Congress party in the national capital.
The just-over-a-year-old party led by Kejriwal emerged as the second biggest outfit with 28 seats, falling only a few seats short of the right wing BJP and allies who managed 32 seats.
The BJP is four short of a majority in the 70-member Delhi assembly. Congress managed only 8 seats.
AAP architect Arvind Kejriwal will be sworn in as the next Chief Minister of Delhi on Dec 28, reports said on Wednesday.
His Cabinet will also take oath on the same day.
--IBNS (Posted on 27-12-2013)