Short answers, pulled from the story.
The Petrograd Soviet held its initial meeting on the 27th of February 1917 at the Tauride Palace. Nikolay Chkheidze chaired this new body with mostly Menshevik deputies.
Bolshevik forces seized power on the 24th of October 1917 when they took control of main telegraph offices and bridges across the Neva. The Second Congress of Soviets opened that evening in the Assembly Hall in Smolny to formalize the transfer of authority.
Non-representative voting gave the Petrograd Soviet almost 3,000 deputies within two weeks after its founding. Most of these deputies were soldiers from the military.
Prince Georgy Lvov initially led the Provisional Government under which the Petrograd Soviet developed dual power. Alexander Kerensky later led that government before the Bolshevik takeover.
The Petrograd Soviet approved the creation of the Military Revolutionary Committee on the 16th of October 1917. Pavel Lazimir chaired this committee with Nikolai Podvoisky serving as his deputy.