Providing Visitor access by default

Hello

We have a new SharePoint 2010 farm with 4 Site Collections.

Currently access is provided by membership to the Visitors group, which in turn contains an AD group "SharePoint-Users".

I'd like to provide this access by default to all users in the company - is it possible to do so without the need for a group? So everyone will have visitor access, then their access to contant will be controlled at individual site level.

Or can I define AD domains in the visitor group...? Coming from a Domino background, the equivalent I'm looking for is the Default access level.

Thanks