DF = Random group, PF = are group aimed to a content with recquirement placed by the leader.
in the DF you have no control on the people that will come into the group (ilevel, class,...). in PF you can decide the ilevel and the class present and much more...

it's why it's stupid to hope for get what you want in DF and quite rude, like i have said is safer to organize a group in PF with people sharing the same goal instead to simply ask to everybody to have the same level of experience and decide to kick a person because he don't match your critera into DF. DF are not your personal belonging.... it's pure random... if you want something more.... like this.... just make a PF.