This analyzes a case that may be more like Asker's. The party arguably subject to joinder (consorité) is a partner or co-owner rather than a parliament.
http://cis01.central.ucv.ro/revistadestiintepolitice/files/n...

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Note added at 3 hrs (2018-07-08 07:24:36 GMT)
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Asker: No, I think everyone's English here and in the Romanian paper is excellent! However, as a native Eng speaker trained as a US lawyer, I find "standing to be sued" is a strange expression which I wouldn't expect to see or use in US litigation. (For instance, the concept is expressly rejected in this decision by a lower US court: https://www.chicagoiplitigation.com/2011/09/party-cannot-lac...
If your Eng document will be used in Switzerland, that may be a different story. But I wouldn't want to recommend an Eng phrase here without first reading at least the materials at https://www.proz.com/kudoz/spanish_to_english/law_general/18... and perhaps more. After more reading and thought I might conclude that your case is the kind discussed in the Romanian paper and that Eng speaking lawyers world would approach it as more a problem of "joinder" or "liability" of the partner rather than "standing." But your whole question may be much easier for some other commenter - maybe a European practitioner - than for me.

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Note added at 3 hrs (2018-07-08 07:44:37 GMT)
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More briefly: in Eng. jurisprudence, standing is a privilege while liability is a misfortune. "Standing to be sued" sounds strange because it says standing but seems to mean liability, like saying someone is "entitled to be punched in the nose." In non-Eng. jurisprudence, the concept of standing apparently is much broader.

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Note added at 3 hrs (2018-07-08 08:01:29 GMT)
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Without seeing the context, it seems like one of the partners in your case is complaining that they should have been joined as a co-defendant or should be allowed to intervene as a cross claimant or third party defendant, because they claim a right to be heard in the matter. In the US we wouldn't view this as a claim that they have a right to be sued or have standing to be sued. No one wants to be sued.

8 . C' est à l' occasion de ce contentieux que les expressions "légitimation active" et "légitimation passive" ont fait leur apparition et ont intégré le vocabulaire juridique communautaire, pour désigner, la première, la capacité du Parlement européen d' être demandeur, la seconde, son aptitude à défendre devant la Cour de justice

8 . It was in the context of the abovementioned litigation that the expressions légitimation active and légitimation passive made their appearance and became part of the vocabulary of Community law, the first referring to the capacity of the European Parliament to be an applicant in actions before the Court of Justice and the second to its capacity to be a defendant .

https://eur-lex.europa.eu/legal-content/EN-FR/TXT/?uri=CELEX...

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Note added at 1 day 46 mins (2018-07-09 05:06:36 GMT)
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a note on eur-lex.europa.eu as source:

whatever is published there is considered as the authentic text of the law in any of the languages of the Member State (it's not merely a "translation provided for information only") so you would expect that everything is triple-checked before publication ...

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Note added at 1 day 51 mins (2018-07-09 05:12:02 GMT)
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of the Member States