No, except in MA and HI (and not always there) an employer is not required to offer health plans to their employees. However, if they choose to offer them, it is not illegal for there to be a requirement that there be 100% participation. In fact, if the company is very small, the insurance carrier may refuse to provide the plan unless all employees participate.
If that is the case, usually an employee can opt out of coverage as long as they provide proof that they have coverage elsewhere.