Trump knew about the meeting in advance with an agent of the Russian government that was there to provide dirt on Hillary. There is your collusion with a side of obstruction of justice and a hint of perjury thrown in by Don Jr.
Source
Michael Cohen, President Donald Trump's former personal attorney, claims that then-candidate Trump knew in advance about the June 2016 meeting in Trump Tower in which Russians were expected to offer his campaign dirt on Hillary Clinton, sources with knowledge tell CNN. Cohen is willing to make that assertion to special counsel Robert Mueller, the sources said.