A party list is a type of electoral system in which political parties create lists of candidates to be elected. The term 'party list' refers to the list of candidates that a party puts forward for election. The SLATE in this crossword solution refers to the list of candidates put forward by a political party. INVITEES, on the other hand, refers to the people invited to the party. In this context, it could refer to the people invited to join the party list.