Groups are stored in the U_Groups field in this format "-2-3-4-".
But I wouldn't recommend deleting users directly like that - leaves too much mess behind in other tables. 6.5 will have a custom search/delete ability. Until then, if it were me, I'd tweak the "delete inactive users script" a bit so it only deleted inactive users that were not part of your member group. That would be a simple one line change and would allow safe deletion.
If you want me to post some code - I can later - what user group number is your paid member group - do this to find out:
SELECT *
FROM w3t_Groups
