Question: "What country was not allowed to be part of the treaty?"  Context: "The Treaty of Hubertusburg, between Austria, Prussia, and Saxony, was signed on February 15, 1763, at a hunting lodge between Dresden and Leipzig. Negotiations had started there on December 31, 1762. Frederick, who had considered ceding East Prussia to Russia if Peter III helped him secure Saxony, finally insisted on excluding Russia (in fact, no longer a belligerent) from the negotiations. At the same time, he refused to evacuate Saxony until its elector had renounced any claim to reparation. The Austrians wanted at least to retain Glatz, which they had in fact reconquered, but Frederick would not allow it. The treaty simply restored the status quo of 1748, with Silesia and Glatz reverting to Frederick and Saxony to its own elector. The only concession that Prussia made to Austria was to consent to the election of Archduke Joseph as Holy Roman emperor."  Answer:
The answer to this question is:
Russia