How can I assist other graduate students who may feel increasingly "unwelcome" due to their countries of origin?
The best thing you can do in my opinion is to be the change you wish to see in the world. Namely, be as friendly and welcoming to the international students as you can be. This will show them that negative views towards people who hail from the countries they come from are not shared by the entire US population, but are simply the opinions of certain people who happen to hold political office these days. It will also make you a role model for other Americans in your lab and department who may be inspired by your positive example to act in similar ways.