To add new users to your Insites account, you must be an account owner.
Click on the icon in the top right corner, and select Users.
Click Invite users and a dialog will appear:
Here you can enter the details for one or more users at once. The drop-down menu on the right of each user specifies what role they have.
When you fill in this form, Insites will send each user an email with instructions on how to create their own account. You can optionally check the Include welcome message box to include your own custom message in that invitation email.