Agreement for Office Boys Staying on Premises
First of all, staying at the office premises is unlawful. You might not have taken group insurance for 24 hours for the employees. If you are still asking your office boys to stay, it may not matter that they will become responsible. You might put an agreement that after office hours they would have to stay at a limited location.
You should safeguard the important documents and assets separately where they do not have access. The agreement should state that they are not permitted to access those areas, and if found entering, as per the code of conduct, the management can make decisions at its sole discretion. They should be provided with one key, with the other held by management, and management can conduct surprise visits at any time after office hours to investigate breaches of the code of conduct.