The general idea behind the pool service is to use uid's from a predefined pool
as a precofigured set of accounts with yet unknown names, so up to the number of
preconfigured accounts getpwnam() calls, happening as a result
of getent passwd username orsu will not fail but will return struct passwd
results matching the name provided as getpwnam argument with the
uid,gid,gecos,dir,shell details preconfigured for the used pool.
Simply compile like specified for nsswitch libraries:
./bootstrap.sh
./configure
make
Then copy the .so.2 file to the location used on your system,
cp ./libnss_pool.so.2 /usr/x86_64-linux-gnu/
Finally, add it to passwd database configuration of nsswitch.conf:
[root@localhost ~]# grep passwd /etc/nsswitch.conf
passwd: files systemd pool