How to associate an existing user to a site in a multisite setup programmatically

Question

In a multisite setup, you can associate an existing user in the network to a site. The standard wp_insert_user function doesn't seem to have the proper logic to handle. How would I do this for multisite?

Also, is there a way to see if a given user is already associated with a particular site?

Answer 1

After looking the code the create user, i found that that I can do:

$userdata = get_user_by('login', 'user1'); 
add_existing_user_to_blog( array( 'user_id' => $userdata->ID, 'role' => 'author');