Close forums but with access to some groups...

[Kungfu]

you can hack it so other people can view but i dont recommend it.

Only other thing would be to make a plugin and when active make it so that all other groups cant browse, basically it looks like its closed to them and if they can they can continue.

you can keep it simple and just make a 2 setting plugin.

Active
Usergroups allowed to browse when active

active being a yes or no radio
usergroups can be a text field and you input like 1,3,5 with commas.

that would be the logic behind it. as far as coding it though im not sure ,you can take a look at the depository and go over the tutorials there.