SynchronizeUsers CB component description
The component's main features are: users synchronization and autologin. To synchronize users you should have the CB installed on main and subsites and the component installed on the main site (including cb plugin and mambot). Once you configure the subsites parameters, the component will start to automatically synchronize users from main to subsites, and if you want to copy users from the subsites there is an option Sub2Main synchronization (but it's manual).
To make the autologin work correctly you'll need: to install the component (with mambot and plugin) on the main site, configure subsites, and make a 'component' menu item. You'll also need to install the component on the subsites (just install, you DON'T NEED to LICENSE it there). When the component is being installed, it automatically installes the mambot and cb plugin.
You'll need the component and the mambot published, so you should unpublish/delete cb plugin on the subsites. Then you may go to the component menu item in the front-end and press the subsite's link. You'll be autologged in there. Find the complete description of the component here.