Per the Wiki, the group API should be configurable via XML. https://docs.jboss.org/author/display/ISPN/The+Grouping+API. However, adding this element (as opposed to attribute – I guess in anticipation of XML configuration of group generators?) results in:

java.io.IOException: unexpected element (uri:"urn:infinispan:config:5.0", local:"groupsEnabled"). Expected elements are (none)
at org.infinispan.config.InfinispanConfiguration.newInfinispanConfiguration(InfinispanConfiguration.java:262)
at org.infinispan.config.InfinispanConfiguration.newInfinispanConfiguration(InfinispanConfiguration.java:280)
at org.infinispan.config.InfinispanConfiguration.newInfinispanConfiguration(InfinispanConfiguration.java:132)
at org.infinispan.config.InfinispanConfiguration.newInfinispanConfiguration(InfinispanConfiguration.java:108)
at org.infinispan.manager.DefaultCacheManager.<init>(DefaultCacheManager.java:253)
at org.infinispan.manager.DefaultCacheManager.<init>(DefaultCacheManager.java:239)
at net.beaumaris.erm.cache.stress.BaseCacheTest.addNode(BaseCacheTest.java:61)
at net.beaumaris.erm.cache.stress.BaseCacheTest.createNodes(BaseCacheTest.java:44)
at net.beaumaris.erm.cache.stress.RehashJoinTest.setup(RehashJoinTest.java:28)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.junit.internal.runners.MethodRoadie.runBefores(MethodRoadie.java:122)
at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:86)
at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:77)
at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:42)
at org.junit.internal.runners.JUnit4ClassRunner.invokeTestMethod(JUnit4ClassRunner.java:88)
at org.junit.internal.runners.JUnit4ClassRunner.runMethods(JUnit4ClassRunner.java:51)
at org.junit.internal.runners.JUnit4ClassRunner$1.run(JUnit4ClassRunner.java:44)
at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:27)
at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:37)
at org.junit.internal.runners.JUnit4ClassRunner.run(JUnit4ClassRunner.java:42)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:49)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: javax.xml.bind.UnmarshalException: unexpected element (uri:"urn:infinispan:config:5.0", local:"groupsEnabled"). Expected elements are (none)
at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallingContext.handleEvent(UnmarshallingContext.java:642)
at com.sun.xml.bind.v2.runtime.unmarshaller.Loader.reportError(Loader.java:254)
at com.sun.xml.bind.v2.runtime.unmarshaller.Loader.reportError(Loader.java:249)
at com.sun.xml.bind.v2.runtime.unmarshaller.Loader.reportUnexpectedChildElement(Loader.java:116)
at com.sun.xml.bind.v2.runtime.unmarshaller.Loader.childElement(Loader.java:101)
at com.sun.xml.bind.v2.runtime.unmarshaller.StructureLoader.childElement(StructureLoader.java:245)
at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallingContext._startElement(UnmarshallingContext.java:478)
at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallingContext.startElement(UnmarshallingContext.java:459)
at com.sun.xml.bind.v2.runtime.unmarshaller.ValidatingUnmarshaller.startElement(ValidatingUnmarshaller.java:90)
at com.sun.xml.bind.v2.runtime.unmarshaller.SAXConnector.startElement(SAXConnector.java:148)
at org.xml.sax.helpers.XMLFilterImpl.startElement(XMLFilterImpl.java:527)
at org.infinispan.config.parsing.NamespaceFilter.startElement(NamespaceFilter.java:51)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:501)
at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.scanStartElement(XMLNSDocumentScannerImpl.java:400)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2755)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:648)
at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:140)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:511)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:808)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:737)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:119)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1205)
at org.xml.sax.helpers.XMLFilterImpl.parse(XMLFilterImpl.java:333)
at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:211)
at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:184)
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:120)
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:103)
at org.infinispan.config.InfinispanConfiguration.newInfinispanConfiguration(InfinispanConfiguration.java:254)
... 28 more