A car manufacturing organisation organises formal, regular meetings with its tier one suppliers for the purpose of collaborating on improvements and changes to the final product. What is this arrangement known as?

This is a supplier association—the key clue in the question is "formal, regular meetings." A supplier improvement project would be a one-off initiative rather than ongoing formal meetings. A supplier forum is more informal than what is described in the question. A supplier focus group meets to discuss specific topics rather than engaging in regular collaboration on improvements. (See p.191)