Two or more persons may sue or be sued as co-parties under the following circumstances:
1.When the rights or obligations of the claim are common to them;
2.When the rights or obligations of the claim arise from the same factual and legal grounds;
3.When the rights or obligations of the claim are of the same nature and arise from the same kind of factual and legal grounds, so long as all defendants are domiciled within the jurisdictional boundaries of the same court or a court can be determined in accordance with the provisions of Articles 4 to 19 inclusive to exercise jurisdiction over all of the defendants.

A third person may intervene to assert a claim against the parties to an action pending in a court of either the first or second instance under any of the following circumstances:
1.When such third person asserts rights to the whole or a part of any claim of the action;
2.When such third person claims that the outcome of the action will infringe its rights.
The provisions of all subparagraphs of Article 56 shall apply mutatis mutandis to an action initiated in accordance with the provision of the preceding paragraph.

Except as otherwise provided, an act conducted by a co-party or by the opposing party against one of the co-parties, and all matters concerning such co-party, will have no effect on the other co-parties.

Wherever a claim must be adjudicated jointly with regard to all co-parties, the following subparagraphs shall apply:
1. Any act conducted by one of the co-parties in the interest of all co-parties will be effective with regard to all of them; any act conducted by one of the co-parties against the interests of all co-parties will have no effect with regard to all of them.
2. Any act conducted by the opposing party against one of the co-parties will be effective with regard to all of them;
3. Any reason for a stay of proceeding, either by operation of law or by a court ruling, that arises with regard to one of the co-parties, will take effect with regard to all of them.
When one of the co-parties, as described in the preceding paragraph, petitions for an appeal, and one or more of the other co-parties are persons who have been declared requiring assistantship, the provisions of Article 45-1 Paragraph 2 shall apply mutatis mutandis.

Wherever a claim must be adjudicated jointly with regard to several persons, and they shall join as indispensable plaintiffs to initiate the action, but if one or some of them refuse to join as co-plaintiffs without giving a justifiable reason, the court may, on plaintiff's motion, order by a ruling such persons to join within a designated period of time. If such persons fail to join, they shall be deemed to have joined as co-plaintiffs.
The court shall afford any persons refusing to join an opportunity to be heard, prior to making its ruling in accordance with the provision of the preceding paragraph.
In accordance with the provision of the first paragraph, wherever the whereabouts of an indispensable person is unknown and that person has not joined in the litigation action, and the plaintiff has moved for joinder of such person, if the court considers such motion just, it may join said person as a co-plaintiff. Notwithstanding, the court may revoke the original ruling where the joined plaintiff submits reasons for refusing to join as a co-plaintiff prior to the first oral-argument session and the court considers such reasons just.
An interlocutory appeal may be taken from the rulings provided in the first and the preceding paragraph.
In cases provided in the first and third paragraphs, if plaintiffs shall bear the litigation expenses, the court may, in its discretion, order only those plaintiffs who initially initiated the action to bear such expenses.

Each co-party has the right to continue the action.
Prior to designating a court session, the court shall notify all co-parties to appear.