To "intervenir" in proceedings generally requires that the party concernded needs to apply to the Court to be added as a party in the case.
I don't think it's wise to assume that this will be as a plaintiff or defendant, as it entirely depends on the situation, for example a party may be added as a third party and any of a number of alternatives.

This is what is known as "Joinder" - (it's also an excellent 7-letter word for Scrabble if you're ever fortunate enough to have the letters. I did once and my co-players threw up such an argument as to whether the word existed...)

Adding Parties and Claims - LawShelf Educational Media
https://lawshelf.com/courseware/entry/adding-parties-and-cla...

Joinder is a process by which parties and claims are added to an ongoing lawsuit. The typical litigation scenario begins with a plaintiff who enters into a lawsuit by suing a defendant. The plaintiff has a claim against the defendant for which he or she seeks some type of relief.
Practice Direction 19A - Addition and Substitution of Parties
https://www.justice.gov.uk/courts/procedure-rules/civil/rule...

1.3 The application to add or substitute a new party should be supported by evidence setting out the proposed new party's interest in or connection with the claim ...

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Note added at 4 hrs (2018-05-30 19:51:29 GMT)
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las partes procesales - ocw@uca
https://ocw.uca.es/mod/resource/view.php?id=1094

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by A Álvarez del Cuvillo - ‎2008
El principio de igualdad de partes: la regla general en Derecho Procesal es que el .... mera exigencia de intervenir en el proceso a través de un representante; ...
¿Quién interviene en un juicio? - Arag
https://www.arag.es › ... › quien-interviene-en-un-juicio

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21 Aug 2014 - Partes interventoras en un juicio dependiendo del procedimiento. ... En un juicio pueden intervenir varias personas dependiendo del tipo de ...

Or "indispensable party". I'm fairly sure that this is the meaning here; the figure is often referred to as a "tercero perjudicado" in Mexico, although in this case, it refers to "Amparo" proceedings in which the necessary party's circumstances are aligned with the government's case.

I don't believe it means a "plaintiff" or "claimant" at all.

The idea in this case, I believe, is that the licensor may sue an infringer of the rights it holds, and for that purpose it may also require its licensees to act as "necessary parties" or be joined to the suit as "indispensable parties" since their interests may be affected ("perjudicado") by the outcome.

El tercero perjudicado es quien, en términos generales, resulta beneficiado con el acto que el quejoso impugna en el juicio de amparo y tiene, por lo mismo, interés en que tal acto subsista y no se destruido por la sentencia que en el mencionado juicio se pronuncie. Por ello debe ser llamado a dicho juicio y tener en éste la oportunidad de probar y alegar en su favor. Podría decirse que hace causa común con la autoridad responsable, que también se empeña en que el acto que de ella se combate quede en pie.
http://mexico.leyderecho.org/tercero-perjudicado/

An indispensable party (also called a required party, necessary party, or necessary and indispensable party) is a party in a lawsuit whose participation is required for jurisdiction or the purpose of rendering a judgment. In reality, a party may be "necessary" but not indispensable. For example, if s/he claims an interest in the litigation, that interest may be impeded if s/he is not joined. That doesn't transform him or her into an indispensable party unless her absence threatens some other party's interest. Often, an indispensable party is any party whose rights are directly affected by disposition of the case. Many jurisdictions have rules which provide for an indispensable party to be joined (brought into the case as a party) at the discretion of the judge. In some cases, the inability to join such a party means that the case must be dismissed...

In patent law, for example, a patent owner is an indispensable party to a patent infringement suit brought by an exclusive licensee against an alleged infringer. The patent owner's rights would be directly affected by a finding of invalidity or unenforceability of the patent claims. At the same time, if the patent owner is not a party to the case, the alleged infringer could be sued separately by the patent owner, and could end up having to pay two judgments for the same act of infringement.
https://en.wikipedia.org/wiki/Indispensable_party

The above definition differs from the one below, in that in the Wikipedia article, they consider the two terms "necessary" and "indispensable" synonymous, though they then clarify that a party may be "necessary" without being "indispensable". In the one below, the difference between a necessary party and an "indispensable" one is defined:

A necessary party is a person or entity whose interests are at stake in the outcome of a lawsuit, whose absence as a party in the suit prevents a judgment on all issues, but who cannot be joined in the lawsuit because that would deny jurisdiction to the particular court. A necessary party who is not in the suit differs from an "indispensable party," who must be joined if the lawsuit is to proceed, and from a "proper party," who could be joined but is not essential.

So, by the definition above, a party can only be "joined" if it is an "indispensable party", but not if it is a "necessary party". Obviously, not being a lawyer, I can only go by the limited research I've done here, and jurisdictions obviously may differ.



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Note added at 8 hrs (2018-05-30 23:25:46 GMT)
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Sorry, forgot the link to the reference above:
https://definitions.uslegal.com/n/necessary-party/

Off the top, of my head, FWIW...