Are they a foreign national that is a NASA employee and sent as part of their job? If so, then absolutely NASA is paying.
As for the employees of other space agencies, speculation on my behalf here, a quick googling of NASA documents didn't reveal anything. But I would not think that NASA is paying for flights, why would they?.
As examples, For 2021, the European Space Agency has a budget of $2.283 billion (USD) and the Japanese Space Agency has a similar budget, they would be budgeting for the cost of astronaut stays years in advance, including their share of the taxi ride, resupply missions, years of training etc. They'd also be paying for a share of maintenance costs for the ISS, tracking and communications and costs of whatever experiments they choose to send up there and conduct.
Thanks for the replies, and yes, there still seems to be a lack of clarity from NASA. The US govt has always been very generous w/TAXPAYERS money, so the issue is discerning. I and many others here I think surely hope we are NOT giving free rides. Russia for sure seemed to make money on their 'rides'.
I can’t say for certain however I believe so. If you remember in the time between the retirement of the Space Shuttle on July 21, 2011 and Crew 1 which launched on November 15, 2020 The only way for American Astronauts to get the ISS was on the Soyuz. The US government paid the Russian government for a seat on the Soyuz. Based on that I can only assume that Foreign countries would also be paying SpaceX and / or NASA for a seat on Dragon.