Redmine

hi,
i need to have in role page a dropdownlist with group selection.
the persons in this group will be assignable in a demand by the person who have this role.
ex:
A is a client.role client and group assignable ruby
this client create a demand and can only assign this demand at groups ruby.
in the group ruby, we have only person who manage ruby dev team.(Chief Ruby_A,Ruby_B)
Ruby_A have a role management_ruby, he create under task and can assign this under task at groups dev_ruby.

I have installed netbeans, i don't know ruby language but i can working on it.
I just need that anyone explain me the simple way to do that.

Excuse for my english, i'm french(i know, it 's not a reason)