Why is there an exclamation mark over a contact?

The exclamation mark over the contact profile indicates that the contact is unverified. The unverified contacts are the contacts who have not activated their email address or have never logged into Freshdesk. 

You can resend the activation emails for these contacts by clicking on the "Send Activation email" button in the Contacts Details page and they can verify their account by clicking on the activation URL in the email.