For your members:
Many people do not file their taxes every year. Not only do they leave money on the table but with how CRA is they may not get all of their money due them. So many of your workers have numerous T4 slips from various companies. How the payroll system is set up source deductions are taken.
The following is related to CPP over contributions:
There is a cap on the amount of CPP a taxpayer has to pay annually. If they surpass the amount required it is usually returned to them at tax time. There is a new stopgap for multiple year filers. If the return they file is more than 4 years after the end of the tax year being filed they will no longer get the overcontribution made for that year or any other previous years. This can have a big impact on not only their refunds, but whether they have to submit installments etc.