To do this you need to design the database with one command table that handle all information required for diferent services. For example you will have a table_user to keep username, password, email, date etc for a user that can be used in blog, forum etc. You will then have one table for each of the feature, such as forum posts, forum topics, blog topic, chat etc.

It is not much special on the database side, but it is critical to program your script to query the databasea and table from different services. if you are using phpbb forum and xyz blog, then you can either modify the signup form of one of these to satisfy both services (one form for two services), or create a whole new registration form that cover the requirements of both services.