Try this:
GraphServiceClient graphClient = GraphServiceClient.builder().authenticationProvider( authProvider ).buildClient();
Group group = new Group();
group.description = "Group with designated owner and members";
group.displayName = "Operations group";
LinkedList<String> groupTypesList = new LinkedList<String>();
group.groupTypes = groupTypesList;
group.mailEnabled = false;
group.mailNickname = "operations2019";
group.securityEnabled = true;
group.additionalDataManager().put("owners@odata.bind", new JsonPrimitive("[ \"https://graph.microsoft.com/v1.0/users/26be1845-4119-4801-a799-aea79d09f1a2\"]"));
group.additionalDataManager().put("members@odata.bind", new JsonPrimitive("[ \"https://graph.microsoft.com/v1.0/users/ff7cb387-6688-423c-8188-3da9532a73cc\", \"https://graph.microsoft.com/v1.0/users/69456242-0067-49d3-ba96-9de6f2728e14\"]"));
graphClient.groups()
.buildRequest()
.post(group);