To allocate a booking to a user's account, select the booking option For a venue user.
Next, either select their name from the Venue users drop-down (for existing users) or select Create a new venue user (for new users). If you choose to create a new user, you will be prompted to provide their details. If you have user-notifications enabled, new users will be sent a welcome email along with the usual booking-confirmation email.
Once a booking has been allocated for a particular booking holder (user account), that booking holder cannot be changed. If you wish to change the holder for an existing booking, it's best to remove the booking and recreate it for the desired holder.