Goldman Sachs Group Inc. hadn’t been formally tapped as a financial adviser by Tesla Inc.’s chief executive officer when Musk revealed plans last week to take the automaker private and said he’d secured the funding for the transaction, according to people with knowledge of the matter. The bank still hadn’t officially signed on when Musk said on Twitter late Monday that he’s working with Goldman Sachs and private-equity firm Silver Lake as financial advisers, the people said. Such a statement from a public company CEO typically signals a formal agreement.