You do not need to enroll in Medicare part B if you or your spouse are still working and are covered by a health plan that is issued through your employer, your spouses' employer, or a union. This only works though if the health plan that is offered by your employer, spouses' employer, or union is the primary payer for healthcare expenses. You will still need to enroll in Medicare B if Medicare A is going to be the primary payer.
You can talk with the employer's healthcare plan administrator to see if their healthcare coverage will be the primary payer or the secondary payer. Businesses with less than 20 employees are deemed small businesses and Medicare A is usually the primary payer. Clear up any questions about what you should do before your enrollment date. If you are not enrolled in Medicare part B and you need to be you will be penalized.