Oct 7 (Reuters) - Life Time Group Holdings Inc touched a market value of $3.3 billion on Thursday after shares of the operator of fitness centers opened 8% below their offer price. Shares of the company, backed by private equity giant TPG, opened at $16.57 per share, down from its initial public offering price of $18. (Reporting by Niket Nishant in Bengaluru; Editing by Maju Samuel) Our Standards: The Thomson Reuters Trust Principles.