If you have multiple businesses, first select the business you want to add new users and then click on "Profile and Services" section from the left panel.
A Drop-down menu will open up by clicking on the "Business Account" option from the top right corner. Click on "Users" option from the drop-down menu to add new users to the business.
If the user you added is already using Refrens then the user can access your business by using his login credentials.
If the user you added is new to Refrens then a mail is sent to the added user to set his password.