If you have other employees who may have been exposed, you should notify them of the possible exposure immediately. By letting them know that an employee has been diagnosed they can gauge their own risk.
Remember that medical information must be kept confidential per the ADA, so it’s important not to disclose which employee was diagnosed. If your employees figure it out on their own that is ok, but make sure you are not the one to divulge that information.
Per the CDC, by following the Public Health Recommendations for Community-Related Exposure, most workplaces should instruct potentially exposed employees to stay home for 14 days, telework if possible, and self-monitor for symptoms.