Try Strengths Finder 2.0 (http://strengths.gallup.com/110440/about-strengthsfinder-20.aspx). This is an assessment tool that rank orders people's strengths and classifies those strengths into categories. For about $10 per person, each member of your group can take the assessment and learn his or her top five strengths. These inform what motivates a person, what their best role in a group is, whose strengths complement whose the best, etc. I use Strengths Finder 2.0 in the context of divorce mediation - as a tool to inform divorcing couples about how the other thinks, operates, reacts, etc. to facilitate healthy communication dynamics during the mediation and beyond.
There are also commonly held ideas about what motivates people based on their generation. For example, people born before 1945 work differently and have different motivations than people born after 1981. I can provide more details on this and the Strengths Finder 2.0 if you like - just call.