mutlisite user site list

so i use a multisite network. when a new user register, from his dashboard>my sites> he is registered to the main site only. even from admin dashboard>users> i can see he is registered to the main site only.
yes, he can login with the same credentials into the subsites, there is no problem with account management. but he can’t quickly surf to another subsite from his admin bar or dashboard.
i want him listed to every subsite.