I know we have forced some employees into employee groups using HR55.2. I need to find out the table name where those employees are stored and eventually forced back into PGEMPLOYEE.

You could use the datafile text for PGEMPLOYEE. Find out what it is related to, what updates it, etc.

The EMPLOYEE table is where those employees are stored.

Records entered on HR55.2 are written to the file PGSELECT (DBHRPGS if you're iSeries).

I hope that's what you were looking for.