The United States opens the tournament June 12 against the Czech Republic in Gelsenkirchen and plays its second game June 17 against Italy at Kaiserslautern.
Reyna is playing for Manchester City in the English Premier League while Donovan, Mastroeni, and Pope are playing in Major League Soccer.
Olsen is the only player on the roster who did not play in any of the United States' 59 World Cup qualifiers.
"This is a special moment for me and I'm looking forward to representing my country in Germany next month," Olsen said.
Swim Week Miami Beach 2014 [PHOTOS]