(a)    Definitions. For the purpose of these rules the terms “party” and “parties” shall include the petitioner, the child, the parent, the guardian ad litem where appointed, the custodian, and every person upon whom service of summons is required by law.

(b)    Other Parties. The state attorney’s office or the Department of Children and Family Services may become a party upon notice to all other parties and notice to the court. The court may add additional parties.