BuddyPress.org

There are several routes to do this – it depends how complex you want the interface to be.

Your simplest option – in your /wp-admin: set the default user role to ‘contributor’ or ‘author’ in Super Admin – Options – Dashboard User Default Role

They’ll then be able to contribute posts to the main blog.

Under their blog tab – you can write a custom post loop that fetches posts where they are the author.