Who can lie to the United States Congress?
The easy answer is, everybody can: they just need the nerve to do so, and the willingness to face the consequences if they are caught (usually this involves perjury, a criminal offense).
The complete answer is that members of the Congress are allowed to lie with impunity in the House of Senate.
The Speech or Debate Clause in Article I, Section 6 of the U.S. Constitution says that Senators and other representatives "shall not be questioned in any other Place" "for Speech or Debate in either House".
This means that, even if it can be proved that some Senator's statement was false ("Of course he lied! His lips moved!") there is no way to punish him.
Politicians should be warned that this privilege ends as soon as they step outside of the Senate: the same statement, delivered with impunity to fellow Congressmen, can bring a libel lawsuit if repeated in a press conference.
Standard disclaimer: Kids! Don't do this at home! Lying to Congress should be left to professionals.
IANAL, and I'm not American either, so some terminology can be incorrect. Call your local Congressman if you want to modify something, or /msg me.