We have 5 employers, each treated for payroll purposes (Forms
W-2, etc.) as separate employers. These employers are members of a controlled group. All 5 employers are covered by one fully-insured health insurance plan. Individually, each of the employers has approx 40 covered employees but the plan itself covers approx 200 employees. Is a Form 5500 required for the group or is there no filing requirement since each employer has fewer than 100 participants?

If your 5 employers are a controlled group you file as one entity and count all participants.

Just to clarify - you have 5 employers who form a control group? So you have common ownership and just one entity sponosrs the plan?

Yes, there is a holding company that is listed as the plan sponsor.