Currently (at v10.1) it is not possible for an editor to related multiple selected features to a single table record without having to drill-down to each feature in the attribute editor and then choose 'Add Selected'. This is not efficient where a large number of features are required to be related to a single table record.
For example, a single owner owns multiple (hundreds of) land parcels - so the editor needs to relate these by selecting the record in the OWNER table and selecting the land parcels on the map (in the LANDPARCEL feature class). The editor currently has to go to each feature in the attribute editor drill-down to the related table, right-click and choose 'Add Selected'.
It would be much better (and I was expecting this function to exist already) if the editor could either:
1) Right-click on the selected table record, choose the related feature class and then choose 'Add Selected' to relate any selected feature on the map to the currently selected table record.
2) Select a record in the table, then in the attribute editor select multiple features, right-click and choose 'Add Selected'
Without customisation there currently appears to be no way to acheive this efficiently from an editor perspective. It is surely a relatively simple change, as the underlying relationship model already exists.