The newest (and youngest) member of the Giants is Javier Alexander Francisco Estrella. He's a 16-year old shortstop from the Dominican Republic. Here's the rule governing eligibility:
Foreign-born players from the Dominican Republic, Venezuela, Mexico, Panama, South Korea, Taiwan, Australia and other countries are free agents. They may sign with any Major League club during the international signing period, which begins July 2 and runs through mid-June the next year. To be eligible to sign a contract, a player must be 16 years old at the time of signing and turn 17 years old by either 1) September 1 or 2) the end of his first professional season, whichever is later.