Most states require a small business to have at least two employees to qualify for group health insurance. Typically, 2-50 employees constitutes a small group, 50-300 is a mid-size group, and over 300 is a large group. The group size can affect the type of benefits available in the group health insurance plan.
More recently, some states have legislated the addition of a category called a "business of one", which allows a small business with only one employee to have access to group health insurance. This makes it possible for the sole proprietor to have the same health insurance benefits as a small group. Colorado is one example of a state that recently allowed this provision.
The State Department of Insurance Contact Information List gives the phone numbers and websites for each state, and can tell you whether your state allows a one person business to qualify for group health insurance. The cost for group health insurance is usually considerably higher than individual health insurance, but the primary advantage is that it allows pre-existing conditions to be guaranteed. If the sole owner of a business does not have any pre-existing conditions, individual coverage is probably the least expensive for similar benefits.