You can easily access it via Settings -> User add more people to the system. Click on the left bar on "Invite new user". Then fill in the displayed fields and assign the desired access rights to the user.
There are three options for access rights. As Administrator the user sees all options in the app (e.g. for operations managers). With Full access the user can view everything except billing data and organisational settings (e.g. for foremen). With the Restricted access the user cannot make any important changes independently but can only view and process tasks that have been preset for him (e.g. for seasonal staff).