When federal regulators late last year accused one of the world’s most popular cryptocurrency platforms of illegally selling $1.38 billion worth of digital money to investors, it was a pivotal moment in efforts to crack down on a fast-growing market — and in the still-nascent industry’s willingness to dive deeply into the Washington influence game. The company, Ripple Labs, has enlisted lobbyists, lawyers and other well-connected advocates to make its case to the Securities and Exchange Commission and beyond in one of the first big legal battles over what limits and requirements the government should set for trading and using digital currency.