President Trump has been impeached for a second time, in January 2021. Now the trial goes to the senate. Now that the Democrats have control of the senate, can they compel witnesses to give testimony, and can that include Donald Trump. Up to 20 January, can Trump use Executive Privilege to avoid being called (both himself and others), and does that change after 20 January?
The senate can certainly subpoena witnesses if they chose to do so in such a trial. They may or may not choose to.
Trump could claim to be exempt from answering particular questions on the ground of Executive Privilege (EP). The exact limits of EP are not well established, but they do not seem to extend to any and every question that might be asked. A claim of EP is probably weaker for a former President than for a sitting President. If such a claim was made in such a trial, the Chief Justice would rule, subject to a vote by the Senate. If Trump still refused to answer after such a claim was denied by the ruling, a citation for Contempt of Congress could be filed for prosecution.
Trump would also be able to raise a Fifth Amendment reason for refusing to answer some questions. This would be plausible, because inciting to insurrection is a crime, as is inciting to riot, and such answers could possibly be used in prosecuting such a crime.
In any case Trump's actions can be determined from media coverage and the evidence of many witnesses, so there seems little need to compel him to testify. Only questions of his motives would require his personal testimony.
Indeed the senate could proceed based solely on published accounts without hearing any witnesses, if it should choose to do so. The facts of who did what are not in serious dispute, only the legal and political implications of those facts, and the appropriate response.