How do I join a community site on the WPMU network?

Question

Social media

It is necessary while maintaining the general registration of users throughout the network (there is by default) to view content and interact with individual sites of the network, you must send a request to the administrator of this subdomain site to join. Those. like joining a community on a social network. For example, in the example.com network, a user registered on site1.example.com to view the content of site2.example.com must click the "join" button and / or fill out the join form (optional). Also with admin ability to change privacy levels.