How to fetch members from particular access group using server side API
One answer
If you can not see the methods of IAccessGroup you are doing it wrong. Follow https://rsjazz.wordpress.com/2015/09/30/learning-to-fly-getting-started-with-the-rtc-java-apis/ and set u your workpace following the extensions workshop.
com.ibm.team.process.common.IAccessGroup.getMembers()