What is client role in enterprise edition?

You can only add a client if you are the project owner/manager for an organization.  (this feature is only available for Organisation/Enterprise plan users)

The client account can be created at the project level

To add/remove client to project

1.Goto project-->Collaborators

2. click on 'Add client' 

3.Enter email id and click on invite/share invite link to the client

Once the client is added to the project, can have the ability to access

Messages: can add messages and reply to added messages

Comments: Can add comments/feedback and convert comments to tasks