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Giving any user admin rights requires modifying the system software. This process is called rooting, as root is the Linux admin account. How you root a device depends on which device you have - despite claims some make, there is no one tool which works for all devices. But the N7 is easy: just visit our Nexus 7 (2013) forum and look in the All Things Root subforum and you'll find what you need.
I don't know about granting some users root access, since I haven't used multiple accounts and root is unofficial and so there's no reason for the OS to support it that way. But ask in that forum, because once people have root they can do a lot of extra stuff.