Our program works like this:
An employee has a major life event (serious illness, etc.) that they do not have enough leave time accrued to cover. The employee goes to our Human Resources Administrator to explain the situation. The Human Resources Administrator discusses the matter with our City Manager. If the situation is deemed worthy of implementing the policy, a resolution is presented to our City Council for approval.
Once approved by the City Council resolution, employees are then notified that they can contribute a maximum number of hours to the employee in question. (We are a very small entity so we pretty much know what the situation is when it is announced). Employees complete a form that is turned into our Payroll division and the leave hours are transferred to the employee in need. The employee receiving the donated hours is not advised of who has donated the hours.
We've used this policy on two occasions in the 10+ years I've worked here: once for an employee who was battling cancer (and unfortunately died from the disease) and once for an employee whose daughter was paralyzed in a car accident.
I'm not sure what the participation rates were for employees in the leave sharing program but I do know that the employees who received the benefits of it were extremely grateful for the generosity of their coworkers so I think it has been rather positively received.