When a group has already some members, the command groupmod raises an error :
java.lang.UnsupportedOperationException
at java.util.AbstractList.add(AbstractList.java:131)
at java.util.AbstractList.add(AbstractList.java:91)
at java.util.AbstractCollection.addAll(AbstractCollection.java:305)
at org.nuxeo.ecm.shell.commands.user.GroupModCommand.modifyGroup(GroupMo
dCommand.java:166)
It is because we try to modify a singleton list which is immutable.