I agree with @AcchaBacha , As for I also know same that you can bring family back before 6 month with out paying any amount. But if the single day is up from 6 month and less than a year than you have to pay 500 Qar per person.
The condition if the time is passed more than a year then RP will got automatically expired.
So if you want to keep their RP valid than try to bring them before one year of period.
for further details you can visit the immigration department.