According to Skanda Purana 1.3b.16 this happened after Shiva commanded his 2 creations - Vishnu & Brahma - to worship him as per the Agamas:-
Viṣṇu and Brahmā wore matted hair and applied sacred ash over their bodies.... They themselves became preceptors, and performed the rites of initiation etc. to the people there belonging to all the castes in accordance with their merits.... Thus Brahmā and Viṣṇu propitiated Aruṇa-Śaṅkara for sixteen thousand years and attained Śivajñāna.
So Vishnu & Brahma became GURUS of some kind. Was this any existing Shaiva sect? If so, which one / what was its name? If it was a new sect started by them - what was the name of that Shaivite sect? Please give sources!