§1. The following must be called to a diocesan synod as members of the synod and are obliged to participate in it: 1/ a coadjutor bishop and auxiliary bishops; 2/ vicars general, episcopal vicars, and the judicial vicar; 3/ canons of the cathedral church; 4/ members of the presbyteral council; 5/ lay members of the Christian faithful, even members of institutes of consecrated life, chosen by the pastoral council in a manner and number to be determined by the diocesan bishop or, where this council does not exist, in a manner determined by the diocesan bishop; 6/ the rector of the diocesan major seminary; 7/ vicars forane; 8/ at least one presbyter from each vicariate forane, chosen by all those who have the care of souls there; also another presbyter must be chosen who, if the first is impeded, is to take his place; 9/ some superiors of religious institutes and of societies of apostolic life which have a house in the diocese, chosen in a number and manner determined by the diocesan bishop.
§2. The diocesan bishop can also call others to a diocesan synod as members of the synod; they can be clerics, members of institutes of consecrated life, or lay members of the Christian faithful.
§3. If the diocesan bishop has judged it opportune, he can invite as observers to the diocesan synod other ministers or members of Churches or ecclesial communities which are not in full communion with the Catholic Church.
Source: Code of Canon Law (Vatican.va)