The Kurdish YPG, and indeed all the major Kurdish factions within Syria, were not invited to the recent rebel confab in Saudi Arabia, but as one of the factions closest to the US, and the dominant force in the new “Syrian Democratic Forces,” they are hoping to eventually have a seat at the table for the Syrian settlement talks.
As one of the most powerful opposition factions in Syria, and the one that holds the most territory (not including al-Qaeda or ISIS), the YPG says its military weight obliges the international community to allow it to participate in the talks.
At the same time, Turkey is heavily involved in those talks and considers the YPG to be a terrorist organization. They’ve attacked YPG forces at times. The Saudis are also keen to center the rebel talks around Sunni Arabs factions they are more closely allied to, hence not inviting the Kurds to the talks.
Russia has backed the Kurds in airstrikes in Aleppo Province, and the US has been heavily backing them in Hasakeh Province, as well as reportedly embedding commandos in Kurdish forces there. The fact that the Democratic Forces are going public with their demand to be involved in the talks, however, suggests support from the US and Russia may not be enough.