If an individual did not sign up for Part B (outpatient services) when first eligible, they face a delay in enrollment. They will need to reconnect with Social Security (the administrative arm of Medicare) during January thru March to ask for Part B and then will be allowed to have their Part B effective July 1st, incurring a penalty of 10% on their Part B premium for each year they were not on Part B when first eligible (carrying that penalty in perpetuity).
If they have an effective date of 7/1/12, they need to make their selection/submission of either a Medicare supplement and stand-alone drug plan or Medicare Advantage (hmo) plan ASAP for a 7/1/12 enrollment. The cutoff for accepting enrollments will differ by carrier. If they miss the submission deadline, they will not be able to have their next election period until Annual Enrollment, when their selection would be effective 1/1/13. If they miss the deadline, they will be on Medicare only and incur responsibility for the deductibles/co-insurances that Medicare does not cover.