Kathleen Tarr says AT&T Inc. employees looked to her as “their de facto 401(k) expert.” Visiting their homes and offices, she advised them on their retirement plans as they called up balances on computer screens. Actually, Tarr worked for Royal Alliance Associates, a brokerage firm owned by insurer American International Group Inc. She encouraged hundreds of departing AT&T employees to roll over their retirement money into the kind of risky high-commission investments that Wall Street’s self-regulatory agency warns against on its website.