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. Current subscribers can trade in their devices for phones enabled with Wifi connectivity when they sign up for the companys early upgrade program.