The group included two heavyweights in former warlord Jean-Pierre Bemba and ex-provincial governor Moise Katumbi.
The talks will take place at a hotel in central Geneva and are expected to last through the weekend.
They aim at bridging rifts that could hand victory to President Joseph Kabila's chosen successor, Emmanuel Ramazani Shadary, on December 23.
Initial "talks have already begun on the sidelines, which is where the real meeting takes place," said Adolphe Muzito, a former prime minister who has joined the opposition.
Four opposition leaders have been authorised by the DRC's election overseers to contest the ballot.
They are Felix Tshisekedi, head of the long-standing UDPS opposition party; Vital Kamerhe, a former speaker of the National Assembly; legislator Martin Fayulu; and former finance minister Freddy Matungulu.
Katumbi and Bemba, like Muzito, have been barred by electoral authorities from taking part but wield enormous influence in their respective regions.
"Everything is open, anything is possible, there is no favourite," Fayulu told AFP about the choice of a single candidate.
He added that an opinion poll last month that put Tshisekedi ahead of Kamerhe, with 36 percent to 17 percent, with 16 percent for Ramazani Shadary, was "not serious."
The poll was jointly conducted by BERCI consulting, a Congolese organisation, and the Congo Research Group at New York University.
The December 23 elections are critical for the future of the DRC, a sprawling, mineral-rich state that has never experienced a peaceful transition of power since it gained independence from Belgium in 1960.
Twenty-one candidates are standing in the presidential vote, which will be combined elections for parliament and provincial assemblies.
Kabila, 47, has been in power since 2001 at the helm of a regime with a reputation for corruption, incompetence and abuse of rights.
His second and final elected term in office ended nearly two years ago, but he stayed in office thanks to a caretaker clause in the constitution.
Months of speculation over his intentions, marked by protests that were repressed at a cost of dozens of lives, ended in August when he threw his weight behind Ramazani Shadary, a hardline former interior minister and loyalist.
On October 25, opposition parties agreed in Johannesburg to name a joint candidate by November 15.
One of the challenges facing them is to overcome differences over the use of electronic voting machines -- a technology that critics say invites electoral fraud.
Fayulu has threatened to boycott the elections if the machines are used, while Tshisekedi has said he will take part whether the machines are used or not.
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