Does anyone have familiarity with auditing group / consortium buying organizations (i.e., multiple companies come together to pool their procurement purchases to obtain greater economies of scale)?  Any particular risks or watch-outs that would be unique from a typical procurement function?

Yes, I have a reasonably good understanding of various aspects for auditing the consortium of buying organizations. It involves reviewing the financial and operational processes, controls framework, and transactions of the consortium to ensure compliance with relevant regulations. It will also ensure accuracy of financial reporting and effectiveness of internal controls.

The design of audit procedures for auditing group or consortium of buying organizations should be done covering these aspects.  

1. Reviewing procurement policies and procedures to ensure compliance with regulations and internal controls. 

2. Testing the effectiveness of internal controls in place to prevent fraud and errors.

3. Reviewing vendor contracts, purchase orders, and invoices to verify accuracy and compliance.

4. Analyzing the consortium's purchasing patterns and trends.

5. Conducting interviews with key personnel involved in the buying process.

One will get an idea of potential risks after testing of internal controls framework that governs the buying function of the consortium. The specific review can then be done for those potential risk areas.  

Due to anti competition laws and adverse effects of cartelization, large organizations refrain from any sort of consortium buying. I have seen that smaller organizations do consortium buying to achieve economies of scale but there can be many inherent risks involved like payment default, price rigging, government penalties etc.