I'm an sqlplus beginner. So I think this is really important info. Someday soon I'll be embarrassed I even bothered to write this.
Log onto the EBS database as the APPS user.
Then do this:
SQL> SPOOL /tmp/userlist.txt;
SQL> SELECT USER_NAME FROM FND_USER;
SQL> SPOOL OFF;
You'll get a nice list of all the User ID's on the database. The "SPOOL" command will print the output to the file indicated, so you don't have to do a screen scrape.
If you want everything in the table substitute '*' for 'USER_NAME'.