Permission to answer topic to other group's forum but only create topic in own group forum

I hope i'm writing this post in the rights section... i asked this question on the general post installation forum but no body gave me an answer... i will try to explain the best as i can.

I'm working on a project of social community, they ask me to create 5 group structures that i made with Organic Groups. I also had to make a leader for each group, so i made a role leader and a role user to manage that.
The problem is that now i have to make a forum with a sub-forum for each group and give permissions to create topic and post to the users of the forum's group. The user of others forums groups can reply to topics but cannont create topics on other group forum. I tried for many days to create that kind of permission, but i don't find the forum permissions on the organics groups options... can some one give me a clue to solve that forum's permissions structure?

....................................Role Leader................Role Users........................Forums
Exemple:-------------------------------------------------------------------------------------------------
OG Group : Spain..............Jhon..............Peter, Maria, Claude...............Forum Spain
OG Group : Germany........Mick..............Lucas, Marco, Matias........... Forum Germany
OG Group : Italy................Paul...............Jean, Carl, Delia....................Forum Italy

Jhon can moderate all topics and post on Forum Spain.
Mick can moderate all topics and post on Forum Germany.
Paul can moderate all topics and post on Forum Italy.

Jhon, Peter, Maria and Claude can create topics and post on Forum Spain but only reply to post on Forum Germany and Forum Italy.
Same structure for other groups.