Can a Canadian company who has a US office establish a retirement plan for the US employees in the US office? The US office employees are US citizens who receive US income and the US office is not a subsidiary of the Canadian company.

Yes, but get a tax pro to figure out whether and how to get the Canadian company a US tax benefit for contributions to the plan.

Thanks for your response. Is there anywhere that you can recommend where I can get something in writing to show that the canadian company can do this?

No. The Canadian company has a branch in the US and presumably withholds US tax from its US employees. I assume it has a US medical plan and other US-based arrangements for its employees. Establishing a savings/pension plan is no different.