Perhaps my views differ a little from the earlier suggestions. In my opinion, if your company is registered under the Factories Act/Companies Act, then there must be its own Certified Standing Orders or it must be following Model Standing Orders, as the case may be. These Standing Orders contain the do's and don'ts for each and every employee employed in the company to which they apply. In some companies, there is a separate set of rules for executives, which they call Conduct, Discipline, and Appeal (CDA) Rules.
Once an individual has accepted the offer of appointment, it is deemed that they have accepted and agreed to the terms and conditions of employment as well, whether or not they have separately signed a document of acceptance. There is one line given in the appointment letters that "accepting the offer of appointment shall mean that you accept the terms and conditions of employment and shall abide by them." Even if this line is not there, joining after the offer of appointment means the same. I strongly recommend making a mention of this statement in future offers if it is not being done at present.
Regards,
AK Jain