For GM Employees & Family Member Discounts, there are several rules.
Here's the most recent published by GM:

Which GM vehicles are eligible for the employee discount program?
Almost every new and unused 2007, 2008 and 2009 GM passenger car, SUV, van and light-duty truck is eligible. To see a list of excluded vehicles, visit the home page and select Vehicle Eligibility from the Vehicle Shopping drop-down menu. Some other models may have limited availability. Eligibility of GM vehicles may change at any time without notice.

Who is eligible for the GM employee discount?

• Spouse
• Children
  
• Stepchildren
  
• Grandchildren
  
• Stepgrandchildren
  
• Grandparents (including in-law and step)
  
• Parents
  
• Stepparents
  
• Siblings (including full, half and step)
  
• Mother-/father-in-law
  
• Sons-/daughters-in-law
  
• Brothers-/sisters-in-law
  
• Same-sex domestic partners (SSDP) of eligible participants also qualify, where applicable.
  
• (Note: Eligible SSDPs are treated the same as spouses and therefore, their eligible family members are also included.)
  
• For more information, contact the National Benefits Center at 1-800-489-4646.
  

As an eligible participant, can I title a vehicle in the name of a living trust?
Yes, you can title a vehicle in the name of a living trust as long as the living trust bears the name of the eligible participant.

Can I share employee pricing or Supplier Discount for Friends pricing with government employees?
Individuals who are employed by a government organization may not be eligible to participate due to relevant government rules. Consult GM Corporate Policy: PPC-06 SALE, DONATION OR COURTESY LOAN OF VEHICLES TO GOVERNMENT OFFICIALS/ENTITIES.

What is the policy for misuse?
Authorization numbers may not be sold or bartered. This program is subject to audit and verification, and misuse will result in sanctions against the GM employee. Violations of these program rules and guidelines by any purchaser may result in sanctions against the eligible participant. Sanctions may include, but are not limited to, the loss of privileges, repayment of monies and/or employment action, up to and including the termination of employment. In addition, legal action may be commenced for violation of program rules.

Can my family member combine this authorization number with current incentives?
In most cases, yes. Have your eligible family member visit a GM Dealer for details on eligible incentives, or you can click here to view current incentive offers.

How many vehicles is my family member eligible to purchase/lease?
The participant may purchase or lease one eligible vehicle per authorization number.

Can I combine my GM Card Earnings with my New Vehicle Purchase Program/GMS discount?
Only Earnings from the GM Extended Family Card (1-800-419-6698) and GM Business Card (1-800-446-5347) may be combined with the employee discount. Please visit gmcardmemberservices.com or call the numbers listed for further details.

What is the GM Supplier Discount for Friends?
Individuals who receive GM Supplier Discount for Friends pricing authorization numbers can enjoy the same preferred pricing that GM suppliers enjoy.

Who is eligible to share the GM Supplier Discount for Friends pricing?
GM active and retired employees.

Whom can I give my GM Supplier Discount for Friends authorization numbers to?
You can share authorization numbers with extended family or friends who are not currently eligible for your GM Employee Vehicle Purchase discount. When deciding on people to share this opportunity with, think about people who don't currently have a GM vehicle, and GM owners who you know want to add another GM vehicle to their household.

Can my referral receive GM Supplier Discount for Friends pricing from any GM Dealer?
This program is available through all participating U.S. GM Dealers and is voluntary on the part of the dealers. Dealer participation is on a vehicle-by-vehicle basis. Your referral should inform the dealer of his or her intention to use a supplier pricing authorization number during his or her first contact with the dealer. If the dealer elects not to participate, your referral may visit another GM Dealer.