Dear Tito_007,
HR is no mandatory pr compulsory to be required by law. The couple of Managers who been managing those 6 years must be broadly aware of the main areas of employment law that affect them and their staff. Are they aware of the same?
Some companies recruit Managers and train them in the fundamentals of Human Resource Management. The extent (of training) to which is relevant for them to convene the work in the organization is certainly suffiecient.
You asked me this question out of your interest, or your organization is now seeking an HR?
If the company is on the look out for a HR, it is profitable -- the induvidal is potential to carry triumphant workforce. I'm not in contradiction with the above.
Managers can be trained on HR Policies & Porcedures applicable to their company's predilection on some specific & detailed requirements than over-arching legal HR requirements. They could be provided with an handbook so that information can be checked or even online would be ok as the infomation can be available to them updated.
They have been running the show past 6 years, sure to the fact they are skilled to perform. Speak to your Management and get the drift to their suggestion & follow accordingly.