Green’s Dictionary of Slang

(US drugs) a person who aids and supports someone having a psychedelic drug experience.

1982		AS LVII:4 289: A guide does not use drugs during parties but serves as a controller, known variously as groundman, babysitter, tour guide, [...] travel agent, or just sitter.
2000		Selene ‘Unusual Mixed Party MDMA’ on ‘Experience Vaults’ on Erowid.org 31 Oct. 🌐 Had a great ‘tour guide’ who supported me and was always there for me and my safety/health.