Ali ibn Abi Talib, Muhammad's cousin and son-in-law, was not at the Saqifah. He was occupied, alongside other close relatives, preparing Muhammad's body for burial, and Shia and Sunni sources broadly agree he was not part of the deliberation that chose Abu Bakr. What they do not agree on is what that absence means. Sunni tradition reads the Saqifah as a legitimate, if improvised, act of communal consultation that produced a rightly guided successor; the fact that the wider community, Ali included, eventually pledged allegiance to Abu Bakr closes the question rather than leaving it open.
Shia tradition reads the same absence very differently: as evidence that a decision of this consequence was made without the one man Shia sources hold Muhammad had already designated, at Ghadir Khumm, as his rightful successor. Ali's own pledge of allegiance did not come until roughly six months later, after Fatima's death, and some Shia accounts describe considerable pressure, including a threat against his household, brought to bear on him in the meantime, a specific claim Sunni sources reject in that form even where they acknowledge Ali's delay. The disagreement is not really about what happened in the hall on that single day; it is about what a rightful succession requires, kinship and prior designation against communal consensus, and that argument, opened at the Saqifah, has never fully closed.