Go to Users submenu from Settings module.
From here you can add user by clicking on Purple and also you can set an user as main user by clicking on the circle.
Fill in the blanks as you wish, also you can use Generate Password to be more secured.
If you want you can opt for sending username and password to email.
Set the hourly/daily rates and costs and then Save.
To edit an user you have to click on user's Small and choose Edit. From here you can change every informations as you wish. Also you can see the Access Rights. When you're done click on Save.
Go to Setting module then to Users submenu and click on Groups.
From here you can add a group by clicking on the Purple then give a name to the group and select the Access Rights. When you're done, scroll down and click on Save.
To edit a group you have to click on group's Small and choose Edit. Change the informations as you wish then Save.
To delete a group, you just have to click on Small then choose Delete.