CONNECTEDUSERS 
Using the
system table
CONNECTEDUSERS, you can determine the following database information:
- All currently logged-on users and the terminal used by the user in question
SELECT username, termid
FROM DOMAIN.CONNECTEDUSERS
Name of the terminal that the user logged on to
SELECT termid
FROM DOMAIN.CONNECTEDUSERS cu, DOMAIN.
CONNECTPARAMETERS cp
WHERE cu.session = cp.session

All defined users: see
USERS