Fiqh of Du'a ?

To make Du'a in any form (e.g. loudly, softly, collectively, etc.) and any time (before, after Fardh Salaat, etc.) is permissible.

While there are substantiations for the different forms and times of making Du'a, Shari'ah does not restrict the making of Du'a in those forms or times not specifically substantiated in Shari'ah. At most, making Du'a is permissible and an act of great virtue and merit.

To regard the collective Du'a after Fardh Salaat or Sunnah Salaat as compulsory will be Bid'ah as that projects the incorrect image of Shari'ah.