Can this be Done,
If i make new usergroup(who can view forum) and add google bot to it.
Will my threads be displayed on google.
You can do that, but you also have to update the usergroup for the bots at:
ACP > Configuration > Spiders & Bots
Just to let you know, if a smart guest searches and clicks "Cached" next to your link, they will be bale to see the thread.