I am assuming that what you are actually looking for is guidance on developing a program to qualify registrars to be used by your association's member organizations based on their competency to provide third-party audits of facilities that these organizations operate. If this is the case, then the answer is that there are no materials that I am aware of, guide book or otherwise, that you could use to identify acceptable registrars for your association's members.

I will be checking on whether such guidance or other material is available, but in the meantime I can suggest the following:
1. Consider whether your association's member organizations all engage in the same types of activities (e.g., aerospace manufacturing). If so, you should identify the Standardized Industrial Classification (SIC) Codes or the NACE Codes that these organizations are covered by. The reason is that registrars are accredited by most accreditation bodies to conduct registration assessments within certain SIC or NACE Codes (NACE Codes differ from SIC Codes in how they group activities but there are equivalencies). If you were able to identify a subset of codes that cover the activities of your association's members, you could use that subset to identify those registrars accredit to offer third-party services for the members (and exclude those that do not).
2. Contact your association's members to ask those that are already registered which registrars they used, what their reaction to the registrars were and whether they would recommend the registrar they used to other association members. Referrals are a good way to go.
3. Verify what type of accreditation your association would value. For instance, if your association's members have facilities located in one specific country, they might want to use registrars accredited by that country's accreditation body. Likewise, if the members have facilities involved in international trade, certain accreditation bodies (e.g., RAB, RvA, UKAS) are more widely recognized in many countries.
4. For which standard or set of requirements are registration assessments being sought? Some registrars provide only quality management system or environmental management system auditing, which could be a factor.
5. Where are the association members located? It is sometimes cost-effective to use a registrar based closed to an organization's facilities, since this reduces the costs involved with having auditors travel to conduct audits.
6. What are the critical issues that your association wants to focus on in recommending guidelines for members to use in selecting registrars? Depending on the issues, you may want to "interview" a number of registrars to see which suit the philosophy and auditing approach of your association's members.

I hope these points help you get started. I will let you know if I find out about any materials you might be able to use. In the meantime, I hope others will respond if they are aware of materials or other criteria you can suggest to your members.