§1. A person who has an interest can be admitted to intervene in a case at any instance of the litigation, either as a party defending a right or in an accessory manner to help a litigant.
§2. To be admitted, the person must present a libellus to the judge before the conclusion of the case; in the libellus the person briefly is to demonstrate his or her right to intervene.
§3. A person who intervenes in a case must be admitted at that stage which the case has reached, with a brief and peremptory period of time assigned to the person to present proofs if the case has reached the probatory period.
Source: Code of Canon Law (Vatican.va)