You can invite more users to your OnINBOX Manager account as long as they have an account with either G Suite or Office 365.
To invite a user, you can do so by going to your profile page in OnINBOX Manager at the top right hand side as shown below.
Under User Management, type in the email address of the user you would like to invite. They will then receive an invitation email with a link to follow and signup. You will also receive an email informing you that an invitation has been sent out to the user.
Once the user logs in to OnINBOX Manager, they will be able to see all the information that you are able to see.