securely exposing sql server 2008 for windows authentication
The words "securely", "exposing", "outside" and "database" do not belong together.

Under no circumstance should you ever expose a SQL server to the internet.  It's a very bad idea.

So your question really should be: How do we give access to our SQL server to remote locations?

The answer: VPN.  The remote users should establish a VPN connection to your network.  From there you can give them the appropriate access to the database.