Generally, when it comes to health benefits, you can cover whomever you want to cover. The most important thing to remember is that once you have established your rules, you must apply them consistently.
An employee does not necessarily have to be physically at work in order to be considered to be actively at work, as it is defined for purposes of health plan eligibility. Otherwise, employees who go on vacation would risk losing their health plan.
Must you cover part-time workers? Generally, you don't have to, though you may if you wish. One thing you should remember is that if you don't set any minimum hours per week or per year that employees have to complete to remain eligible for health benefits, you cannot terminate their coverage when, for example, they go on a disability leave. To avoid that problem, some employers who want part-timers to participate will establish a rule that says that any employee who works, for example, at least 30 hours per week is eligible to participate in the health plan. You should discuss your alternatives with your health insurance provider.
Do you have to offer coverage to spouses or dependents of workers? In short, no. If you do, you should clearly define who is a dependent.
May you offer benefits to unmarried domestic partners? You may, of
course, but if you do, you should be aware that the fair market value of the
insurance paid will be taxable to your employee under federal law. Also, it can
be very difficult to define who is a domestic partner — for example, how long
must the relationship have lasted? If you're interested, talk to your insurance
company about it.