Adding sites to a super admin profile

Hi there. I am running a network of six sites on WP, and have gone into the list of super admins within Users on the Network Admin screen. I wish to increase some site access to a few super admins, but when I go into their user profile, there seems to be nowhere I can simply add sites to the existing suite of sites they have access to. Anyone know how I can do this?

Also: there is a user who has a profile on site x. I want to add his profile to site y. On site y, I searched for his profile; it doesn’t exist. Yet when I go to create a new profile for him on site y, it says his username and email already exist (presumably because his profile is recorded on site x). How do I migrate his user profile across to site y?

Cheers.