An emploiyee often signs only an employment contract before they are in the first day of their employment. A contract says that an "emplyee handbook" is a part of teh contract but the employee may or may not receive this huge (sometomes 500 pages) document before they actually start. You can't blame a person for not reading about "griefing procedures" when they don't have anything to relate these to.
To the OP:
Owning a part of a business (which in the present is almost necessarily equity stocks) is a fundamental human right and an employer cannot deprive you from this.
However, they will likely make it more difficult to excercise. In the extreme case you may be:
1) Prohibited from anything that may carry risk of you loosing more than the value of the trading account: shorting stocks, buying on margin, trading any kinds of derivatives (options, futures etc).
2) Prohibited from anything which looks different from investment acvtivity (for example, owning a stock for less than 15 days)
3) Required to obtain a permission from a compliance officer to buy/sell any stock on the day you execute a transaction
4) Required to execute trades exclusively through the firm's brokerage (assuming they own one).
I think the easiest way to sort out your dobt is to mention to whoever is your contact in the company that yopu own a few stocks and want to find out about their compliance policy. They may either send you a compliance manual or arrange a 5-minute telephone call with a compliance officer.