who are you, and what is this in reference to?

I'm a commercial user.

The plugin needs to be licensed - without licensing the code explicitly, it can't be (legally) used by anyone. Someone needs to put a LICENSE.txt file or similar holding the proper license (e.g. MIT, GPL, BSD, etc.) for the plugin into the GitHub repo.

Legally, since this code has no explicit license attached to it, it's licensed solely to the authors, with no rights to copy or use it granted. It needs a license doc.

Please consult with the overall project on this.

This is a problem specific to this particular plugin, not the Jenkins project as a whole.

Other plugins have similar LICENSE files and/or describe the license in the pom.xml file in their respective GitHub repos.

I can't make any decision or push for anything on this issue - there's literally nothing for me to consult on. This problem specifically has to be handled by the people who are responsible for writing/maintaining the Join Plugin code; they own the copyright on the code, and need to decide (and publish) what license they want to make it available under.

https://wiki.jenkins-ci.org/display/JENKINS/Before+starting+a+new+plugin

"If nothing is defined, your code will be assumed to fall under the MIT license terms."

That link isn't good enough, mainly because it's never referenced in the plugin documentation, nor in the GitHub repo anywhere. That's a pretty obscure place, especially since it's not something that any end-user is going to look. It's also not legally binding, which is the concern here - code has to be explicitly licensed, not by inference.

really, this is simple. Add the following to the pom.xml file:

    <licenses>
      <license>
        <name>The MIT license</name>
        <url>http://www.opensource.org/licenses/mit-license.php</url>
        <distribution>repo</distribution>
      </license>
    </licenses>

Or, heck, just check-in the LICENSE.txt file obtained from one of the other plugins.

E.g. https://github.com/jenkinsci/backup-plugin/blob/backup-1.6.1/LICENSE.txt

Guess you'll have to make the risk decision here.