Clicking a user thumbnail shows the options you can apply to that user:
Open/Close Shared Workboards
Opens the Workboards Pane (if not already open) with the selected user’s shared workboards.
You can access the workboards of a user if that user has shared them with you. In the same manner, other users can access your workboards if you have shared them with those users. The Sharing option from the options menu in the Workboards Pane is used to define access permissions for a workboard or a folder.
Clicking the Open/Close Shared Workboards option again closes the user’s list and displays the public workboards on the Workboards Pane.
Add As Friend
Adds the user to your list of friends. The friendship indicator appears in the upper-right corner of the thumbnail.
Clicking the Remove From Friends 
option removes the user from your list of friends.
Subscribe
Subscribes you to this user so that you can follow the workboard actions the user performs. The subscription indicator appears in the upper-right corner of the thumbnail.
Clicking this option again removes the subscription.
Start a Discussion
Clicking this option starts a discussion with the user. It displays the Discussions Pane (if not already open) and opens a message in a new discussion.