What is the ruling on raising one's hands and supplicating privately, then concluding it collectively and audibly with: "Exalted is your Lord, the Lord of might, above what they describe," after the prayer?

This act includes a legitimate matter, which is supplicating after the prayer and raising the hands during it, and another illegitimate matter, which is doing so in congregation. Collective supplication and remembrance after the prayer have no Sunnah to support them.