What is the difference between being a member of the organization and being a board member of the organization? The same question can be asked about the difference between a shareholder and a director? The biggest difference is liability. A shareholder is not held accountable for the activities of the organization.
If you are a shareholder of Facebook you are not liable for their bad actions, if you sit on their board you could be held liable when things go wrong. The same thing applies to members, if you are a member you have no liability for the organization’s wrongdoing, if you are a board member the accountability lies with you. The simplest example I see is when Canada Revenue Agency asks a board member to pay the source deductions personally for the organization. This would never happen to a member.
If you are being asked to join a board, you would be wise to see what additional liability you are taking on before you make that decision.