A Sample Suite Listener which can retrieve the names of all methods that are part of a group.

GroupPrinter.java

public static class GroupPrinter implements ISuiteListener {

    @Override
    public void onStart(ISuite suite) {
        String groupName = System.getProperty("groupName", "all");
        List<ITestNGMethod> allMethods = suite.getAllMethods();
        List<ITestNGMethod> filteredMethods = new ArrayList<>();
        for (ITestNGMethod method : allMethods) {
            if (Arrays.asList(method.getGroups()).contains(groupName)) {
                filteredMethods.add(method);
            }
            if (Arrays.asList(method.getBeforeGroups()).contains(groupName)) {
                filteredMethods.add(method);
            }
            if (Arrays.asList(method.getAfterGroups()).contains(groupName)) {
                filteredMethods.add(method);
            }
        }
        System.err.println("**** Filtered Groups " + filteredMethods);
    }

    @Override
    public void onFinish(ISuite suite) {

    }
}