It is possible to bulk edit a selection of clips. First of all, select the clips that you would like to edit.
Click on the 'More actions' button and select 'Bulk update'.
In the list, choose 'tags'. You can also bulk edit other fields here (such as Camera, Card name), as well as custom fields (such as the first "Yes/no").
You can now either choose an existing tag by either typing it or selecting it from the list, or you can make a new tag by typing it in the field.
Click "Save" and the tags will be added to all your selected clips.