There are two questions that need answering here:
1.) Is it a group plan?
2.) If it is a group plan, what does the plan say about who is eligible.
IF it is a group plan AND IF you are eligible as defined by the plan document, then you must be given a chance to enroll. IF it is a group plan AND IF you are NOT eligible as defined by the plan document, then you CANNOT be given coverage.
IF it is not a group plan, then outside of a legally binding contract or CBA that guarantees coverage, it's up to the employer who does and does not get coverage as long as he does not violate Title VII and related laws.