External dependency without group

In https://docs.gradle.org/current/dsl/org.gradle.api.artifacts.dsl.DependencyHandler.html you can find description of two notations for external dependency. There is also information, that:

In both notations, all properties, except name, are optional.

When and how can I use external dependency without group? Can you give me existing examples?

btw, shouldn’t be group: group instead of group: group: in:

configurationName group: group:, name: name, version: version, classifier:
classifier, ext: extension

?

For example:

If you have

repositories {
  flatDir {
     dirs '/path/to/local/folder'
  }
}

and the local/folder contains junit-4.1.jar you can do

dependencies {
   testCompile ':junit:4.1'
}