When you unassign a grid card from a user, grid card moves to the unassigned list and can then be reassigned to another user. You can also filter to see the state of assigned and unassigned grid cards and export unassigned and unassigned grid cards for bulk operations.
To modify, in bulk, the state of existing assigned user grid cards, see Set grid card state.
Note: You cannot unassign a grid if it has been used. You can only unassign grid cards that have never been used (last used date is never).
Assign unassigned 
 grid cards to users
Edit group membership 
 of unassigned grid cards