USA coach Jurgen Klinsmann welcomed Los Angeles Galaxy forward Donovan and four other Major League Soccer players plus Borussia Dortmund forward Terrence Boyd on Sunday in Orlando, Florida, for two-a-day workout sessions.
Sporting Lisbon's Onyewu is set to arrive on Monday, a week ahead of AZ Alkmaar's Altidore, who figures to be in the final 23-man roster picked from 27 hopefuls for five upcoming matches that Klinsmann will select next Friday.
"We are excited now to get towards full numbers with the MLS guys and also the players from Europe who are coming in with great form," Klinsmann said. "We want them to pick up the rhythm as fast as possible and make them understand that we are taking the next couple weeks very seriously."
Donovan is the all-time US scoring leader, while Altidore scored 19 goals for AZ Alkmaar this season. The Americans will play host to Scotland on 26 May in Jacksonville, Florida, and five-time FIFA World Cup champion Brazil on 30 May in Washington before visiting Toronto for a friendly against Canada on 3 June.
US players launch their quest for a seventh consecutive trip to the FIFA World Cup on 8 June when they entertain Antigua and Barbuda in Tampa, Florida, in their first semi-final round match of CONCACAF regional qualifying. Four days later the Americans visit Guatemala.