[Brajesh Singh]

No prob
Please Go to your site. When you are logged in and are visiting the group directory[Yoursite->groups] you will see an option to create Group.
create group and you will see the option to enable/disable forum while creating group.
If you have existing group, go to group page->Click on Admin button, In the settings submenu you will see the option to enable enable/disable forum.
hope it helps.