When setting up the rules for a Secret Santa exchange, the most important points are to determine who will be a part of the event, what the price range for gifts will be and when the gift exchange will occur. By setting up these rules, everyone can participate if they want to and everyone will receive fair gifts of the same monetary value on the same day.
By sending out a company-wide email or even an invitation via social media, employees at the office can determine personally if they want to engage in Secret Santa. Make sure that the gift price limit is not so high that some employees can't afford to participate. Once it's been determined who wants to participate, make the date of the exchange clear so that no one forgets their gifts.