By Marina Lopes WASHINGTON (Reuters) – T-Mobile US added 2.75 million customers in August, the largest number of monthly subscriber additions in its history, the carrier announced at a company event on Wednesday. Led by quirky Chief Executive John Legere, the third-largest wireless operator in the U.S. While rivals Verizon and ATT networks stretch almost across the country, T-Mobiles network is clustered around urban centers. The move comes the day after Apple Inc. announced it will release an iPhone programmed with technology to allow automatic shifts between WiFi and carrier networks.