Can I legally decide to keep my Medicare and not join my employees health insurance?

Read the text of the waiver you signed. If your employer has more than 20 employees then they must cover individuals age 65 and over on the same basis as those under 65, so they cannot offer coverage secondary to Medicare or any other incentive (such as cash or increased salary) to get you to choose Medicare as a primary plan. The waiver should explain that what you rejected was group coverage with Medicare paying on a secondary basis in favor of Medicare ONLY with no secondary coverage. If the waiver did not properly explain this, then the waiver is invalid. Even if the waiver was appropriate, if your plan has an annual open enrollment period, you should also be able to opt into the employer plan at open enrollment and have Medicare as a secondary payer. If Medicare is your secondary payer, you do not have to pay for Part B (unless you want to) and you will have an opportunity to enroll in Part B without penalty within a short period of time after your employment ends. Call your local Social Security office for details and additional explanation of how Medicare works with employer coverage.