This would be an approach: https://rsjazz.wordpress.com/2012/12/09/analyzing-a-aroject-areas-members-and-roles-using-the-plain-java-client-libraries/

Or you can use the reportable REST API.
https://jazz.net/wiki/bin/view/Main/ReportsRESTAPI#projectArea_type_com_ibm_team_pr

There are multiple ways.. if you are willing to head Plain Java API way, here is all the information you need for it :
https://rsjazz.wordpress.com/2012/12/09/analyzing-a-aroject-areas-members-and-roles-using-the-plain-java-client-libraries/


and if you would like to go for Reportable REST API way :
 
On a Browser Address Bar, provide the following URL :
https://<repo_url>:<port>/ccm/rpt/repository/foundation?fields=projectArea/projectArea[name='<nameofprojectarea>']/(name|allTeamAreas/(roleAssignments/(contributor/name|contributorRoles/name)))

after replacing <repo_url>, <port> and <nameofprojectarea> in the above url with appropriate values from your instance.

Sample Output with :
    <repo_url> = clm602.ibm.com
    <port> = 9443
and
    <nameofprojectarea> = 56458_UserNotPartOfTeamArea
 
would be :

<foundation Version="1.0.0" rel="next" href="https://clm602.ibm.com:9443/ccm/rpt/repository/foundation?id=_s4bFUO7JEeaFpuXOGjoZGA&fields=projectArea%2FprojectArea%5Bname%3D%2756458_UserNotPartOfTeamArea%27%5D%2F%28name%7CallTeamAreas%2F%28roleAssignments%2F%28contributor%2Fname%7CcontributorRoles%2Fname%29%29%29&size=100&pos=100">
    <projectArea>
        <name>56458_UserNotPartOfTeamArea</name>
        <allTeamAreas>
            <roleAssignments>
                <contributor>
                    <name>builduser2</name>
                </contributor>
                <contributorRoles>
                    <name>Scrum Master</name>
                </contributorRoles>
                <contributorRoles>
                    <name>default</name>
                </contributorRoles>
            </roleAssignments>
            <roleAssignments>
                <contributor>
                    <name>builduser1</name>
                </contributor>
                <contributorRoles>
                    <name>Scrum Master</name>
                </contributorRoles>
                <contributorRoles>
                    <name>default</name>
                </contributorRoles>
            </roleAssignments>
        </allTeamAreas>
    </projectArea>
</foundation>

More about Reportable REST APIs is here :
     https://jazz.net/wiki/bin/view/Main/ReportsRESTAPI#projectArea_type_com_ibm_team_pr

hope this helps