Cal-OSHA recently enacted new emergency guidelines that apply to nearly every employer in California. These rules are in effect immediately. An informative Fact Sheet can be found here.
In short, all employers must do the following:

• Establish and implement a written COVID-19 Prevention Program that has detailed information regarding workplace hazards, methods of prevention, and systems for communicating and reporting; This written plan must be available to all employees and if requested by representatives of the DIR;
• Provide training and instruction to all employees on how COVID-19 is spread, on infection prevention techniques and any state, federal and local benefits that may be available to them; and
• Establish protocols for responding to COVID-19 cases that may occur in the workplace and criteria for employees to return to work. A negative COVID-19 test may not be required for an employee to return to work — the employee must be fever free for at least 24 hours, the employee’s symptoms must have improved, and at least 10 days must have passed from the date the employee’s symptoms first appeared.
• If employees are sent home because of an exposure, the employer may be required to continue paying their wages.

In addition, if there is an outbreak (3 or more cases in a 14-day period), or a major outbreak (20 or more cases in a 30-day period), employers are required to do the following:

• Offer testing (either once or twice per week, depending the the level of outbreak) at no cost and during work hours to all employees present at the workplace until there are no new COVID-19 cases detected in the workplace for a 14-day period;
• Provide all employees information on available benefits; and
• Notify local health department within 48 hours.

Employers are also required to track and maintain records for all COVID-19 cases.
The full rules can be found here. FAQs can be found here. And a sample prevention plan can be downloaded here. These rules are rapidly changing and evolving so be sure to review with your employment counsel.