The most importsant question here is do you assign roles or allow people to select? I don't have an answer except if you use some form of persoanalty testing, then use it at least assign roles in the first iterations.

Topics: Persoanlty assessment - yes or no? List the roles Subject Matter Experts (SME) - specific expertise Financial Analyst - business impact Interim Manager - tracking the iterations Solution Owner - represent the solution owner Devils Advocate - forces the group to think through decisions before action ​Conflict Manager