Currently, this is not built-in. The security-roles created in the Explorer demo-data generator are "user" and "admin". It's possible to add your own groups and alter the UI to refuse access to the modeler when the user is not part of the given role.
For such UI-changes, it's required that you alter the files in the activiti-explorer project source code (this is a maven-project, jar) and include your custom activiti-explorer.jar in the explorer WAR instead of the original one.
