How can i remove the register link? I want my forum to be exclusive for my group only.

I don't think you can. But if you want your forum to be exclusive for your group only, the best alternative is to adjust your admin settings so that you have to manually authorise each account before they are allowed to use your forum.

Go to the Administration panel (link "administration" located at the bottom of your main forum page) > "Configuration and appearance of the message board" > "Registration lock" > "Manually accept each new registration"

Thanks buddy.