Seif is one of the country's most prominent opposition figures. He was released from prison in July 2010 after years in jail for anti-government activities.
He was arrested for the first time in 2001 for criticizing Assad and sentenced to five years in prison. He was re-arrested in January 2008 and sentenced to two and a half more years on charges of "weakening national sentiment," a term usually used to mean carrying out anti-regime activities.
Syria had previously banned Seif from travel, a measure regularly taken against dissidents. In 2007, the U.S. State Department urged Syrian authorities to allow him to leave the country to receive medical treatment.
Seif told The Associated Press in 2007 that refusing to allow him to seek treatment abroad for prostate cancer was "like being sentenced to a slow death."
Atassi comes from an influential family from the central city of Homs. She has been an outspoken critic of Assad for years and was arrested days after the uprising began while taking part in an anti-government protest in central Damascus.
The SNC was formed last year but quickly came to be viewed as ineffective and out of touch with activists and rebels fighting a bloody war on the ground.
U.S. Secretary of State Hillary Rodham Clinton harshly criticized the SNC late last month and called for a leadership that can rally broader support, including inside Syria.
After the agreement was signed, the foreign ministers of Turkey and Qatar, Ahmet Davutoglu and Hamad bin Jassim, joined the conference. Davutoglu said claims of divisions among the opposition "are over now."
Tunisian Foreign Minister Rafik Abdessalem said the new coalition "is a major achievement on the road to a new Syria."
U.S. State Department spokesman Mark Toner released a statement saying that America congratulates the opposition and looks "forward to supporting the National Coalition as it charts a course toward the end of Assad's bloody rule and the start of the peaceful, just, democratic future that all the people of Syria deserve."
All opposition groups and figures taking part in the Doha meeting rejected any dialogue with Assad's regime.
The Syrian government has dismissed the meetings in Doha. Information Minister Omran al-Zoubi called them political folly. In an interview on state-run Syrian TV aired late Friday, al-Zoubi said those who "meet in hotels" abroad are "deluding themselves" if they think they can overthrow the government.
The uprising against Assad began in March 2011 with peaceful protests. A regime crackdown prompted fierce fighting, propelling the conflict into a civil war that has taken on sectarian overtones.
In all, activists say more than 36,000 people have been killed.