Staff members should be added to Impero back:drop as part of the initial set up. More information can be found here: How do I set up Impero back:drop?
Once a staff member has been added they will appear in the Staff Members list:
Firstly, use the drop-down menu to assign the staff member to a user group (see How do I manage user groups and permissions?). Use the tick box on the right of the staff member to select them - you can select multiple staff members at once - then scroll up to the top of the screen, select 'Send Welcome Email' from the drop down menu and press Save.
The staff member will then receive an email welcoming them to Impero back:drop, prompting them to set a password to access the system.