We are building the largest open hardware community on the web so 123D Circuits has been build to be social from the ground up. You can collaborate on circuits in real-time, fork and comment to help each other build the best electronics.
As the owner of a circuit you can choose Invite Collaborators from the right sidebar of the Circuit Detail page.
Goto the Project Detail page and choose Collaborators from the menu. Choose Delete Collaborators from the right sidebar.
Choose Discussion in the sections menu to open the comments. You don't need to be a 123D Circuits member to comment.