The rebel group, Islam Army, said in a statement late on Saturday that it had "received an invitation to attend the Riyadh conference which has been decided to be held between (December) 8-10". The statement did not say whether the group planned to attend.
Representatives of two other rebel groups confirmed they had received invitations dated Dec. 8-10.
Saudi Arabia is a strong supporter of opposition groups seeking to topple President Bashar al-Assad, whose future has been a sticking point in attempts at a peace agreement.
The conference will not include Islamic State, the jihadist group that has taken over swathes of eastern Syria and northwest Iraq and declared them as part of a cross-border caliphate. The United States and its allies are waging air strikes against the group in both countries.
Many of the Syrian opposition groups are fighting IS at the same time as Assad. The al Qaeda-linked Nusra Front, listed as a terrorist group by the United States and United Nations, will not be part of the conference either.
Riyadh has not yet given an official date for the meeting. It is also not clear whether the conference would be held in the capital or elsewhere in Saudi Arabia.
Members of Syria`s exiled political opposition have said the meeting would bring together at least 65 representatives of political and armed groups.
Monzer Akbik, a member of the Turkey-based Syrian National Coalition, gave similar dates and said a delegation of 20 SNC members had been invited. The SNC is the main Western-backed political opposition, although it has only tenuous links with rebels on the ground and is seen as out of touch with the general population.
Seven members of Syrian internal opposition group, the National Coordination Body, have also been invited, Akbik said.
Among groups fighting on the ground, 15 were expected to be represented, including Islam Army and the powerful Ahrar al-Sham, he said.
An Amman-based opposition figure said at least 12 rebel groups were expected to be present.
Islam Army, an Islamist group formed by a merger of rebel factions in 2013 and mainly based in the Eastern Ghouta area near Damascus, is led by Zahran Alloush, one of the most prominent figures of the insurgency.
Saudi rival Iran has criticized the opposition conference, saying it would cause the failure of international talks.
Tehran said on Sunday Assad`s fate should only be decided by the Syrian people.
The conference comes after an international agreement to open talks between the government and the opposition by Jan. 1.
The United States said on Saturday the next talks on Syria`s conflict, which has killed 250,000 people and displaced millions, would take place in New York later this month.
Last month, Russia, the United States and powers from Europe and the Middle East met in Vienna and outlined a plan for a political process in Syria leading to elections within two years.
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