Hello!
Concerto is no longer as popular as it used to be. Here and there you can find someone who even has one. Again, he doesn't know if anyone is even putting it on any computer. Perhaps as a virtual machine. There is little chance that someone would want to add it to a stand-alone server, because there will be problems and you'd better give up :-).
If you would like to add yourself to the list of users, this is otherwise not so terribly difficult.
But if you don't have this option (because the previous admin ran away...) you have to deal with the database:
"concerto_production" and entering a new user in the "User" Table record. For this new user, you specify a username (enk e-mail address), password, which must be encrypted (for example password: clevo207 = $2a$10$wYSdc/7Rv9eogXhZpS8.suY86JjyjJ4MaVE13SdvtYloCmlcvPBSu) and whether the user is an Admin.
Otherwise, I don't know how "native" you are in this business, and I also don't know how the Conceto you use is even installed.
I myself use the stand-alone server version and have Concerto installed on Ubuntu 18.04. I also have WEBMIN on the same server so I can access the database through it.
If your Concerto is part of a virtual machine; for example Oracle's VirtualBox, the database needs to be updated manually. or to install some tool for editing the database on such a machine. Webmin somehow doesn't work, but it would work with PhpMyAdmin.
I hope that I have at least indicated the direction of the solution.
Hello, Jure
-- Premisli, če je tole sporočilo vredno izpisa na papir!