> is there any way the db admin may know which databases are in use and
> who are the current users ?

You can download FSQL command line utility from here
http://www.volny.cz/iprenosil/interbase/

First, attach to service manager and execute Show Attachments command.
It will give you list of databases in use.
Then connect to particular database and execute Show Connections
to get list of attached users.

Here is screenshot:
--------------------
SQL> CONNECT SERVICE 'localhost' USER 'SYSDBA' PASSWORD 'masterkey';
SQL> SHOW ATTACHMENTS;
Number of attachments: 3
Number of databases: 2
Database in use: C:\DB.FDB
Database in use: C:\DATABASE\TESTDB.FDB

SQL> DISCONNECT SERVICE;

SQL> CONNECT 'C:\DATABASE\TESTDB.FDB' USER 'SYSDBA' PASSWORD 'masterkey';
SQL> SHOW CONNECTIONS;
Active connections = 3 (SYSDBA, TEST, TEST)

SQL> DISCONNECT;
--------------------

Ivan