You do understand how interveners in Canada and friend of the court (forget how to spell the Latin term) in the US work. These are entities that don't have any actual standing in a case but have an interest in their own right, and they want to be heard by the court. They file a petition with the court to give them the right to be heard in the matter. They don't have any connection with any of the actual parties of the case.
From Wikipedia:
Intervenors are most common in appellateproceedings, but can also appear at other types of legal proceeding such as a trial.
In general, it is within the discretion of the court to allow or refuse an application to intervene. There are exceptions to this however (for example, under subrule 61(4) of the Rules of the Supreme Court of Canada, if the court has stated a constitutional question then the attorney general of any province or territory, or of the federal government, may intervene "as of right", i.e. without the need to be granted leave to intervene).
Courts will tend to allow an application to intervene if the applicant will provide a different perspective on the issues before the court, without expanding those issues.
Intervenors are permitted in criminal matters as well as civil matters. However, courts sometimes express concern in allowing applications for intervention in criminal matters where the applicant will make arguments against the position of the accused. It sometimes is seen as unfair that the accused in a criminal matter be required to meet arguments from sources other than the prosecution.
There are several distinct reasons why someone might wish to intervene in a proceeding:
- if the proposed intervenor is currently a litigant in a case with legal issues similar or identical to the case at hand;
- if the proposed intervenor represents a group of people who have a direct concern in the legal issues raised in a case (for example, if the case involves deportation of a particular individual, an application for leave to intervene might be made by an interest group for the rights of refugee claimants);
- if the proposed intervenor is concerned that the court's decision in a particular case might be so broad as to affect the proposed intervenor's interests; in other words it would be an intervention to ensure that the court's ruling does not have "accidental" unintended effects.
It is often said that the role of intervenors is to "assist" the court in making a just decision on the dispute at hand. While it is true that judges sometimes do indicate that intervenors have been of aid to the court in reaching a decision, the use of the word "assist" can be seen as misleading in that it implies the intervenor is acting altruistically. In general, the goal of the intervenor is to influence the court in making its decision, not just to "assist" the court.
Canadian courts (also courts in UK) use the term "amicus curiae" in a more limited sense. Generally, in Canada, an amicus curiae is someone who has been specifically commissioned by the court to provide a viewpoint which the court believes is necessary and otherwise lacking. By contrast, an intervenor is someone who has applied to the court to be heard on a matter. For example, the Quebec Secession Reference (a case in the Supreme Court of Canada) had one amicus curiae and several intervenors