Distribution of key by server

I have read that in asymmetric cryptography, a system admin can generate key pairs on the server. After that, the users will be given the generated key pairs and this is an efficient way of doing this kind of thing.

I have two questions:

1. 
   

   How can the admin. distribute the key pairs? If say, via email, isn't it time-consuming?

   
   


2. 
   

   I thought private key should be private...but the admin. actually knows it as well?