There’s one called added_existing_user that fires immediately after adding an existing user.
Digging a little deeper, I find the function add_user_to_blog() is used by both add_new_user_to_blog() and add_existing_user_to_blog(), and has an action hook named add_user_to_blog. I’d look into using that last hook, if I wanted to ensure something happened when a user was added to a site, whether they were _new_ or _existing_.