The ATM qualifications are documented in the SCAMPI Method Definition Document (MDD) Section 1.3.2 SELECT TEAM MEMBERS. 

I have extracted the related text here.

Parameters and Limits

• The minimum acceptable team size for a SCAMPI A appraisal is four people (including the appraisal team leader).
• All team members must have previously completed the SEI-licensed Introduction to CMMI course.
• With regard to engineering field experience, the team (as a group) must have an average of at least 6 years of experience, and the team total must be at least 25 years of experience in each of the disciplines to be covered in the appraisal.
• With regard to management experience, the team (as a group) must have a total of at least 10 years of experience, and at least one team member must have at least 6 years of experience as a manager.
• The team must, in aggregate, have representative experience in the lifecycles being appraised.

Optional Practices
Although not required in the Parameters and Limits section above, the following are considered recommended best practices and should be employed whenever feasible:

• Each member should have good written and oral communication skills, the ability to facilitate the free flow of communication, and the ability to perform as team players and negotiate consensus.
• At least half of the team members should have participated in a previous process appraisal.
• Team members should be perceived by the appraisal sponsor as credible.

Additional appraisal team member selection considerations include

• Consider the personal characteristics of individual team members (e.g., communication preferences and personality types) and how these characteristics may affect the dynamics of the team.
• Use one or more authorized SCAMPI Lead Appraisers as team members.

And there is one more ATM requirement that is documented in the MDD errata. The Appraisal Sponsor cannot be an Appraisal Team Member.

Copied with some modifications from: http://ppqc.blogspot.com/2009/07/appraisal-team-member-qualification.html