There are times when you want all of the registered member on your blog network to be members of all the blog. WordPress does it differently. By default all the registered users are added to the main blog with the role you have specified(or “subscriber” by default ). If the user creates a new blog, they are removed from the main blog and added to the new blog they created.

My solution here will allow to automatically add users to all blogs on your network, it does not matter much if they register a new one or they do not have a blog but just a registered member. In case the user registers new blog, this function will do no harm the user will be admin of the new blog as in normal case.

To our help, there are two functions

is_blog_user():-It checks whether the current logged in user is a member of the current blog or not.

add_user_to_blog($blog_id,$user_id,$role):- it allows you to add an existing user to a blog with the specific role where $role can be “subscriber”,”author”,”editor”,”contributor” or “administrator”

Now the code, putting the following code in your theme’s functions.php will do the job or you may wrap it as a plugin in case you want it that way

 
add_action('wp','bpdev_add_user_to_blog',10);//hook our function
 
/*add logged in user to current blog*/
function bpdev_add_user_to_blog(){
if(!is_user_logged_in())
return false;//do not do anything
 
global $current_user,$blog_id;
 
if(!is_blog_user())//check for current membership
add_user_to_blog($blog_id, $current_user->ID, "subscriber");//give subscriber role for current blog
 
}

And if you are using buddypress, you can put the above code in bp-custom.php.

What it does:-

When a logged in user visits a blog on your network, it checks whether he/she is a member of the current blog or not. If not, it adds them as a member to the current blog.

I hope it helps in some of the cases. Let me know it it works for you or not ?