A system for clustering electronic calendar schedules in an event scheduling user interface displays the availability of invitee clusters that each include invitees with sufficiently similar availability during a target period. An invitee cluster list includes an invitee cluster entry for each invitee cluster, such that each invitee cluster entry displays the availability for all invitees contained in the corresponding cluster, as well as the number of invitees contained in the cluster. Invitees that do not belong to any cluster are listed as individual outlier entries, such that each outlier entry displays the availability for the corresponding outlier invitee and the invitee's name. The names of invitees contained in an invitee cluster are displayed in response to user selection of the cluster entry. A degree of similarity required for invitees to be clustered together, and/or a permitted total number of clusters, may be input from a user.