To set up a SIMPLE IRA an employer must have 100 or fewer employees earning more than $5,000 each - including all employees who have worked at any point in the calendar year. And the employer cannot have any other retirement plan besides the SIMPLE IRA.
If your employer offers a SIMPLE IRA, you qualify to contribute if you earned at least $5,000 a year during any two years before the plan was set up, and if you expect to earn at least $5,000 this year.