People are added to the seal by the Uzokage adding their blood to a special part of the seal that they can only access via a specially made ring that they keep on always. There are to backup rings stored in specific places only the Uzokage knows of.
Nothing above A rank is in the Library whether it is a jutsu or secret. If the village is invaded then certain indivuals can self destruct the library.
The first floor is entered by the entrance and any Whirlpool ninja or supervised civilian has access to it. It contains majority of the information on Taijutsu, Weapons, Chakra Theory, Chakra Control, and Healing. It contains the basic information on Ninjutsu, Genjutsu, other nations, and ninjas. It also has E-C Rank jutsus in it.
The second floor can only be accessed by a stairwell on the first floor and only Whirlpool ninja above the rank of Genin can access it. It contains any information above the rank of Genin on Taijutsu, Weapons, Chakra Theory, and Chakra Control. It has majority of the information on Seals. It has majority of the information above the rank of Genin on Ninjustu, Genjutsu, Healing, Other nations, ninjas, and about Whirlpool. It also has B Rank jutsus in it.
The third floor can only be accessed by a stairwell on the second floor and only ablove the rank of Chunin or ninja with special permission can access it. It contains information above the Chunnin Rank on Ninjutsu, Genjutsu, Healing, Seals, Other Nations, Ninjas, and about Whirlpool. It also has A Rank Jutsus. There is of course information not in the Library that the Uzokage decides discretion on.