Staff members should be added to Impero Backdrop as part of the initial setup. More information can be found here: How do I set up Impero Backdrop?
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 then click on Save.
The staff member then receives an email welcoming them to Backdrop, prompting them to set a password to access the system.