How to easily bring 80 MySQL users into ProxySQL?
Since you're using ProxySQL with Percona, there's a nifty tool made by the good people at Percona that does exactly what you're looking for (sync's mysql users to proxysql).

If you install ProxySQL using the Percona RPM, it comes with a nifty script that helps with managing your ProxySQL instance called "proxysql-admin". That script has the option to "--syncusers". Here's an excerpt of the help info for the script:

[root@localhost ~]# /usr/bin/proxysql-admin
Usage: [ options ]
Options:
  --config-file=&lt;config-file&gt;        Read login credentials from a configuration file (overrides any login credentials specified on the command line)
  --quick-demo                       Setup a quick demo with no authentication
  --proxysql-datadir=&lt;datadir&gt;       Specify proxysql data directory location
  --proxysql-username=user_name      Username for connecting to the ProxySQL service
  --proxysql-password[=password]     Password for connecting to the ProxySQL service
  --proxysql-port=port_num           Port Nr. for connecting to the ProxySQL service
  --proxysql-hostname=host_name      Hostname for connecting to the ProxySQL service
  --cluster-username=user_name       Username for connecting to the Percona XtraDB Cluster node
  --cluster-password[=password]      Password for connecting to the Percona XtraDB Cluster node
  --cluster-port=port_num            Port Nr. for connecting to the Percona XtraDB Cluster node
  --cluster-hostname=host_name       Hostname for connecting to the Percona XtraDB Cluster node
  --cluster-app-username=user_name   Application username for connecting to the Percona XtraDB Cluster node
  --cluster-app-password[=password]  Application password for connecting to the Percona XtraDB Cluster node
  --without-cluster-app-user         Configure Percona XtraDB Cluster without application user
  --monitor-username=user_name       Username for monitoring Percona XtraDB Cluster nodes through ProxySQL
  --monitor-password[=password]      Password for monitoring Percona XtraDB Cluster nodes through ProxySQL
  --without-check-monitor-user       Configure ProxySQL without checking/attempting to create monitor user
  --enable, -e                       Auto-configure Percona XtraDB Cluster nodes into ProxySQL
  --disable, -d                      Remove any Percona XtraDB Cluster configurations from ProxySQL
  --node-check-interval=3000         Interval for monitoring node checker script (in milliseconds)
  --mode=[loadbal|singlewrite]       ProxySQL read/write configuration mode, currently supporting: 'loadbal' and 'singlewrite' (the default) modes
  --write-node=host_name:port        Writer node to accept write statments. This option is supported only when using --mode=singlewrite
                                     Can accept comma delimited list with the first listed being the highest priority.
  --include-slaves=host_name:port    Add specified slave node(s) to ProxySQL, these nodes will go into the reader hostgroup and will only be put into
                                     the writer hostgroup if all cluster nodes are down.  Slaves must be read only.  Can accept comma delimited list.
                                     If this is used make sure 'read_only=1' is in the slave's my.cnf
  --adduser                          Adds the Percona XtraDB Cluster application user to the ProxySQL database
  --syncusers                        Sync user accounts currently configured in MySQL to ProxySQL (deletes ProxySQL users not in MySQL)
  --version, -v                      Print version info
[root@localhost ~]#


Notice the "--syncusers" option down there at the bottom. That's what you're looking for, yes? ... and it's made and distributed by Percona. :)

Hope this helps!