What is the ruling on a congregational prayer that prays alone after the first congregational prayer has ended, while refusing to allow another worshiper to join them, and what is the ruling on the one who does that, and are these people considered Shiites?

It is not permissible for someone who enters the mosque to refrain from performing the prayer with the congregation if it has been established. It is recommended for someone who prayed at home to repeat it with the congregation, based on the saying of the Prophet, peace and blessings be upon him: "If you have prayed in your dwellings, then you come to a congregational mosque, pray with them, for it will be a supererogatory prayer for you both." It is not permissible to leave an obligatory prayer after starting it without a legitimate excuse, based on the saying of Allah the Almighty: "And do not invalidate your deeds." The congregation mentioned are disobedient for failing to pray with the congregation when it was established, but their second prayer is valid.