To add multiple EXISTING cards to the board go to the project search 
and filter your search by cards, card category and any fields or hashtags to get the cards you wish to add in a list. Press the little tick 
to select the first card, then the double ticks 
in the top right corner to Select All.
Now you can choose any bulk actions i.e. change card categories, change fields, delete, or bulk add to a board. To bulk add to a board, press the grid 
and then select the board from the drop down menu and Save.