In general, the answer is “Yes,” but there are a few exceptions. If you decide to establish a SIMPLE IRA, every eligible employee must be offered a SIMPLE IRA account.
An employee is eligible if they have earned $5,000 in compensation during any two previous years, and are expected to earn $5,000 the current year. If an employee is unwilling to participate, the employer must open up a SIMPLE IRA on behalf of the employee.
Employees excluded from a SIMPLE IRA are those who are not eligible, nonresident aliens, and union employees who are included in a union retirement plan. However, you can establish certain eligibility requirements for a SIMPLE IRA such as period of time worked for the company, and so on, if they do not contradict the minimum requirements.
These will be outlined in a Summary Plan Description, which must be shared with employees by 60 days prior to the start of each plan year (normally by November 2). It is always wise to consult a professional to determine the rules for SIMPLE IRAs, since after you have established these rules, you cannot change them.